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Lighting Sleeping troubles

Book & Article of the Month (June – July)

If you follow us on social media, you already know what we picked for our monthly reading material. For those who don’t, don’t worry: we’re also highlighting them here, on the blog. Let’s dive in, with…

June’s article of the month

Interindividual variability in neurobehavioral response to sleep loss: A comprehensive review

We picked this article because we think the kind of math we do at Arcascope—coupling biophysics with machine learning—is going to be clutch for capturing inter-individual effects in real world data.

And this is important, because (drumroll):

The big differences from person to person in how they handle sleep loss can make it a challenge to predict an individual’s fatigue risk level. That’s why we need new and sophisticated models to track fatigue—including models that take into account the dynamic, constantly shifting nature of the human circadian clock.

There are other reasons to care about inter-individual differences. Quoth the authors:

“Individual variability in response to countermeasures with different pharmacological targets suggests it may be possible to personalize the selection of countermeasures against the effects of sleep loss using information about genetic variants of implicated receptors.”

In other words, the fact that caffeine and other stimulants work differently for different people means that someday we could be recommending when to drink coffee based on your genes.

We completely agree with the authors that “Research should strive toward a systems approach to the study of interindividual vulnerability to sleep loss in which behavioral, neurobiological, and genetic data are integrated in a larger framework delineating the relationships between genes, proteins, and their functional consequences with observable alterations in cognitive functioning and behavior.”


July’s book of the month

In July, we went in a different direction, reading

Wild Nights: How Taming Sleep Created Our Restless World by Benjamin Reiss.

We really enjoyed this tour of sleep through the lens of history, literature, and society, featuring quotes like:

Did the switches go on because people wanted to stay up later, or did people stay up later because the switches went on?

Light at night has definitely changed the way we live, and most of us aren’t in a rush to go back to 1878 levels of illumination. But the growing evidence that light at night can disrupt your health in a whole host of ways should have us all asking: What can we do differently?

The good news is that there are a lot more ways of improving your circadian health than just keeping the switches off in the evening.

One of the other quotes that stood out to us:

In this age of connection, people might take classes on the other side of the world when it’s 3am in their home time zone. Or they might check their phone at night and be jolted awake by news.

This puts our body clocks, which track the light around us, in conflict with the things that demand our attention, which are running round-the-clock.

We also liked the call-out to how sleep deprivation has had massive, society-level impacts throughout history:

“Sleep deprivation has been blamed for such high-profile industrial and transportation accidents as the Chernobyl nuclear meltdown, the Exxon Valdez oil spill, and the Challenger space shuttle disaster, as well as less spectacular but more systemic problems such as loss of worker productivity, impaired memory, and increased health and emotional problems.”

History has shown that sleep deprivation is not a topic that should be taken lightly. Yet it can still be dismissed as nothing more than just “feeling tired”. We want to change that. With our app, Shift, we are giving people the tools they need to fight sleep deprivation and finally take the “tired” out of their life.

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Circadian science Lighting

What matters besides light?

A Guide to Non-Photic Zeitgebers

We can all agree that the difference between night and day is, well, night and day when it comes to light. The progressive change in light present in our environment is subconsciously tracked by our bodies and that information is used to help time our sleep-wake cycles. Of course, not everyone has the physical ability to perceive the changes in light which occur over the course of the day. Yet some blind individuals are still able to entrain accurately to their environment without these crucial photic cues. How is that so?

Light is the strongest “zeitgeber”— or, environmental cue that provides input to the circadian clock. Our bodies use the signals from zeitgebers to try to synchronize our internal clocks with our environments. For example, the decrease of light over time that you experience while watching a sunset can communicate to your body that nighttime is approaching. If you keep the lights on instead, your body’s clock can be delayed and production of the hormone for night, melatonin, can be suppressed. Because of this, it’s important to think about what signal your light exposure is sending to your internal clock (cough, screens at night, cough). But light is not the only player in town. There are other zeitgebers besides light that influence your circadian time.

Before getting into the weeds, it will help to have a mental map for how your body’s clocks are organized. Our circadian clocks are composed of a central clock and peripheral clocks. The central clock acts as the command center located in the suprachiasmatic nucleus (SCN), sending signals to the various tissue-specific peripheral clocks spread throughout the body. The phase of each peripheral clock is influenced by both the central clock and factors specific to that system, allowing different biological functions to be coordinated through the central clock while being autonomous enough to respond to stimuli specific to that system. For example, as we’ll discuss below, the metabolic clock is driven by both the central clock but is also affected directly by meal timing. Some signals may primarily affect peripheral rhythms, while others can affect both peripheral and central rhythms.

Zeitgebers, such as light, communicate with SCN
Zeitgebers, such as light (a), communicate with the SCN which houses the central circadian clock. Other signals, like meal timing (b) can relay their own signals to the body’s peripheral clocks in organs and tissues (c and d).

Before we talk about these non-light inputs to your body’s time, let’s talk about phase-response curves. The standard way to track how a zeitgeber advances or delays your central circadian pacemaker is through phase-response curves (PRCs). These graphs represent the timing of a zeitgeber stimulus (x-axis) and its quantitative effect on the timing of a circadian biomarker (y-axis), like shifts in melatonin timing or core body temperature minimum. A phase-response curve can tell us when a zeitgeber will advance you, when it will delay you, and when it will have essentially no effect. 

Phase-response curve
Phase-response curve showing circadian time of exercise vs. phase shift, determined by presence of 6-sulphatoxymelatonin (aMT6s) measurements. Participants performed 1 hour of moderate treadmill exercise at the same clock time for 3 consecutive days.

Exercise has been shown to induce central clock phase changes dependent upon the timing of activity. One study in particular, which produced the PRC above, recruited 101 physically active adults to investigate the effects of exercise on circadian phase. Participants were put on a 90 minute ultradian light schedule (60 minutes light/wake followed by 30 minutes dark/sleep) for approximately 6 days. Baseline aMT6s (urinary melatonin) measurements were made for individuals and averaged across the whole sample of participants. The difference between the individual and sample phase was determined and then subtracted from the external clock time of exercise to calculate a “circadian time” adjusted for interindividual differences. 

Each participant performed 1 hour of moderate treadmill exercise at the same clock time for 3 consecutives days following baseline, isolating the effects of exercise on phase. Moderate exercise was quantified as 65-75% of the heart rate reserve calculated for each participant based on their individual maximal heart rate. This study concluded that exercise at 7am and between 1pm and 4pm causes phase advances to aMT6s timing, whereas exercise between 7pm and 10pm provoked phase delays. 

These results can be read from the PRC above by identifying the selected time along the x-axis and reading the corresponding point’s phase shift, quantified by the y-axis. For example, exercising at 7pm (19 h) results in approximately -0.75 h phase shift. The negative value indicates a delay, whereas a positive would indicate an advance. Thus, exercise at 7pm causes a little less than an hour phase delay based on the above PRC. 

A similar pattern of phase advance and delay regions can be observed in the human response to light, especially in studies with parallel protocols or similar duration of stimulus. Intuitively, the analogous behavior of morning advances and evening delays produced by light and physical activity makes sense—light and activity are often correlated, so if they told really different circadian stories, it would be pretty strange. The authors of the exercise PRC research note that the phase shifting strength of exercise is comparable to that of light exposure, making exercise another great tool for your circadian management tool belt.

Phase-response curve showing Dim Light Melatonin Onset
Phase-response curve showing Dim-Light Melatonin Onset (DLMO) determined circadian time of melatonin supplement vs. phase shift. Participants were held on an 3.5 hour ultradian light-dark cycle (1.5 hours dark/sleep period followed by 2 hours light without sleeping) with administration of 3.0 mg of melatonin following the dark/sleep period. 

Oral melatonin supplements have also been shown to shift central clock outputs like dim light melatonin onset. In general, melatonin dosing tends to do the opposite of what light exposure would do at the same time—delaying you when you’d be advanced by light, or advancing you when you’d be delayed by light. While most people don’t think of melatonin as something that can be mistimed, the phase-response curve tells us that melatonin at the wrong time might have no phase shift whatsoever, or could delay you when you’d really prefer to be advanced (e.g. if you want to fall asleep faster at night). 

Meal Timing Study, factors controlled by light.
A visual depiction of results from a meal timing study showing which factors were controlled by light (and the SCN, purple, top), and which were controlled by meal timing (green, bottom). Larger arrows mean more significant control by that input.

Eating patterns are interesting. When you keep light exposure patterns fixed and change the timing of meals, the vast majority of relevant outputs—melatonin, cortisol,  hunger, triglycerides, and genes like PER3 and BMAL1—stick with the patterns set by light (in other words, they follow the SCN). A few of the players track with meal timing, however: glucose, and, to a lesser extent, PER2 and insulin patterns, showed significant phase shifts in response to the food time changing.

This means that misaligning your meal timing with your light timing could result in a kind of desynchrony, in which your light and your food are sending two different time-keeping signals, and in turn throwing your metabolic processes out of whack. This desynchrony—or, specifically, avoiding this desynchrony—could be the reason why time-restricted eating (TRE) has been found to improve cardiometabolic health. 

So light’s not the whole story. On the one hand, that means there are more circadian-relevant factors to have to worry about; on the other, that means we have more knobs to turn as we try to help people achieve optimal circadian health. Can’t get bright light? Why not try exercise instead? Need more than just a light dose? Here’s the right time for melatonin, for you. At Arcascope, we want to use all the tools available to us to help you optimize your external and internal cues, in search of synchronous bliss.

This blog post was written by Carrie Fulton, one of Arcascope’s interns. Thanks, Carrie, for your hard work!

References

  • St Hilaire MA, Klerman EB, Khalsa SB, Wright KP Jr, Czeisler CA, Kronauer RE. Addition of a non-photic component to a light-based mathematical model of the human circadian pacemaker. J Theor Biol. 2007 Aug 21;247(4):583-99. doi: 10.1016/j.jtbi.2007.04.001. Epub 2007 Apr 4. PMID: 17531270; PMCID: PMC3123888.
  • Celine Vetter, Frank A.J.L. Scheer, Circadian Biology: Uncoupling Human Body Clocks by Food Timing, Current Biology, Volume 27, Issue 13, 2017, Pages R656-R658, ISSN 0960-9822, https://doi.org/10.1016/j.cub.2017.05.057. (https://www.sciencedirect.com/science/article/pii/S0960982217306231)
  • Youngstedt SD, Elliott JA, Kripke DF. Human circadian phase-response curves for exercise. J Physiol. 2019 Apr;597(8):2253-2268. doi: 10.1113/JP276943. Epub 2019 Mar 18. PMID: 30784068; PMCID: PMC6462487.
  • Burgess HJ, Revell VL, Molina TA, Eastman CI. Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. J Clin Endocrinol Metab. 2010 Jul;95(7):3325-31. doi: 10.1210/jc.2009-2590. Epub 2010 Apr 21. PMID: 20410229; PMCID: PMC2928909.
  • Burgess HJ, Revell VL, Eastman CI. A three pulse phase response curve to three milligrams of melatonin in humans. J Physiol. 2008 Jan 15;586(2):639-47. doi: 10.1113/jphysiol.2007.143180. Epub 2007 Nov 15. Erratum in: J Physiol. 2008 Mar 15;586(6):1777. PMID: 18006583; PMCID: PMC2375577.
  • Mohawk JA, Green CB, Takahashi JS. Central and peripheral circadian clocks in mammals. Annual review of neuroscience. 2012;35:445. doi:10.1146/annurev-neuro-060909-153128.
  • Wehrens SM, Christou S, Isherwood C, Middleton B, Gibbs MA, Archer SN, Skene DJ, Johnston JD. Meal timing regulates the human circadian system. Current Biology. 2017 Jun 19;27(12):1768-75.
  • St Hilaire, M.A., Gooley, J.J., Khalsa, S.B.S., Kronauer, R.E., Czeisler, C.A. and Lockley, S.W. (2012), Human phase response curve to a 1 h pulse of bright white light. The Journal of Physiology, 590: 3035-3045. https://doi.org/10.1113/jphysiol.2012.227892
  • Kripke, D.F., Elliott, J.A., Youngstedt, S.D. et al. Circadian phase response curves to light in older and young women and men. J Circad Rhythms 5, 4 (2007). https://doi.org/10.1186/1740-3391-5-4

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Circadian science Lighting Sleeping troubles

Light at night is bad, people.

Imagine you’re on a swing on a playground with a friend standing behind you.

This friend is not a jerk, so they’re going to push you when a normal person would push you on a swing—right when you’re at the end of your backwards motion and ready to start moving forward again. They, like a normal person, are going to stay out of your way the rest of the time.

If they push you a little bit late or a little bit early, no big deal. If they push you way early, or way late—like, for instance, when you’re still very much in the middle of swinging backwards— that’s a different story. Imagine having your blissful, carefree swing interrupted by smacking into someone standing right in your path. Imagine them actively pushing you back in the direction you came from, right when you least want to be pushed that way.

That person is light exposure. The case where they give you a boost, speed you up, get you to a bigger swing: that’s light exposure during the day. The case where they slow you down, get in your way, reduce the size of your swing: that’s light at night.

And okay, okay: it’s never that simple. “Day” and “night” mean different things based on your body clock’s current time. Light exposure somewhat slowing down your body’s clock is part of a normal day. The same light can affect different people in different ways.

That said, I love this analogy because it captures something I think about a lot in the context of circadian rhythms. The secret of a good swing is having a clear difference between your forward and backwards motion. Shoves are good in the forward direction, and not-so-good in the other direction. Similarly, more and more it seems to me that the secret to healthy circadian rhythms is having a clear difference between the active and inactive parts of your day. When your body wants light, get lots and lots of light. When it’s time for dark, get the darkest dark you can.

I’m thinking about this today because I just read “Light at night in older age is associated with obesity, diabetes, and hypertension” by Kim et al. in the journal SLEEP. Using a dataset of 552 community-dwelling adults aged 63-84, they looked at how exposure to light at night is associated with cardiovascular disease risk factors. They found significant associations between light at night and obesity, diabetes, and hypertension, but no such associations between average light over the course of the 24-hour day and those same risk factors.

In other words, the fact that it’s at night matters. When you get light at night, you muddle the difference between night and day. You lose that good swing.

Caption: Light (left) and activity (right) in the light at night (LAN, yellow), and no light at night (No-LAN, purple) groups. The LAN group has more light at night as well as lower activity during the day, and less of a difference between day and night.

This paper isn’t the only paper to look at light at night—many others exist—and I don’t want to get in trouble for confusing correlation and causation (the above are correlations only). That said, there are plenty of possible mechanisms by which light could increase your risk of cardiovascular disease.

One is by throwing off your internal clock, so your metabolic machinery is not firing on all cylinders, or is rising and falling in a way that’s mistimed relative to when you’re eating.

Another is by suppressing production of melatonin, which your body produces naturally once a day, but won’t produce if it thinks it’s still light outside. Melatonin has a number of properties that are important for circulatory and metabolic health, so it makes sense that having less of it around may not be the greatest thing for you.

But at the highest level, I find real value in thinking about that swing analogy. There are some things in our body that are meant to be dynamic and strongly rhythmic: breathing, walking, heart rate. Our 24-hour rhythms are no different. Get that good swing by getting lots of light during the day. Keep that good swing by turning them all off at night.

Thanks to the authors of Kim et al. for their great work! We really enjoyed the read.

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Circadian science Lighting Shift Work Sleeping troubles

What do shift workers do & what might they do?

So you want to help shift workers feel better—sleep better, be safer, have fewer of the long term chronic health problems that go hand in hand with shift work. How do you do it? Where do you start?

As some of the most circadian-wrecked people around, shift workers have been the topic of no small amount of research. Yet one incontrovertible, “best” strategy has failed to emerge for what shift workers should do. There are plenty of reasons for this, but the short answer is: it’s complicated! There are a lot of possible shift schedules a person can be on, and a lot of variation from person to person in how those shifts will affect them. In this blog post, I’ll try to chip away at the complexity a bit by covering what’s currently known about strategies for shift work, and what shift workers might do in the future.

Rather conveniently, a lot of the ways you try to help shift workers can be framed as a choice between two alternatives. So let’s start with one of the biggest “versus” there is out there.

Homeostat vs Circadian Interventions

There are two main forces that conspire to make a person feel sleepy. One is your sleep hunger, or sleep homeostat—basically, a build up of “need for sleep” that accrues when you’re awake, and drains when you’re asleep.

Figure 1 from Bailey et al., 2018. The two process model has the sleep homeostatic (red line) getting bigger while you’re awake, and going down when you’re asleep. The entrance and exit to and from sleep is regulated by the body clock (black curves). Waking up in the middle of the night? It might be because your circadian clock isn’t quite aligned to where you want it to be.

The other is your body’s circadian clock, which sends an extra strong signal once a day to tell you to go to sleep. These aren’t the only things that make a person sleepy, but they explain a lot of the phenomena we see in shift work contexts. This way of thinking about sleepiness (homeostat plus circadian) is called the “two process model of sleep.”

You could classify the strategies around helping shift workers into two camps, based on which of these two forces—homeostat or circadian— they’re primarily targeting. If you want to have a low sleep homeostat going into the night shift, for instance, you probably want to sleep as close to before your shift as you can. So you might try staying up until 1:00 pm on the day after your shift, building up a ton of sleep pressure, then falling asleep for most of the afternoon and evening, waking up right when it’s time for work. Naps and caffeine would also fall under the header of “mostly targeting the sleep homeostat.”

Targeting the circadian clock, however, means moving your rhythms to promote sleep at a time you actually can sleep. This means phase shifting your clock, which can be achieved by doing the kinds of activities that matter to the clock (getting light exposure, avoiding light, exercise, etc.) at the right times.

These methods aren’t mutually exclusive by default, but they can be in conflict at times. A lot of what decides that is the direction you choose to move your clock in.

Advancing vs delaying the clock
Two directions for shifting the clock, from Burgess et al. 2002. Large white rectangles correspond to night shifts. Small white rectangles correspond to 3 hour pulses of bright light aimed at delaying (left) or advancing (right) the clock. The pulses move in time because the person’s circadian clock is shifting in response to the signals from the previous days.

A totally day-adjusted person will probably have their peak fatigue hours occur sometime in the early morning; say, 3:00 am. If they go on a night shift, those peak fatigue hours are happening right in the middle of work hours. (Not exactly ideal). So you could shift their rhythms so that their worst hours no longer happen at 3:00 am.

Way #1 to Achieve This: Shift them later, or delay their clock. Move it so they’re feeling the biggest circadian drive to sleep at, say, 9:30 am, after they’re home from work.

Way #2: Shift them earlier, or advance their clock. Move it so they’re feeling the biggest circadian drive to sleep at, say, 5:00 pm, or before they go to work.

Way #1, or delaying the clock, is often called “compromise phase position.” The idea is that it’s a compromise for the night shift life—you’re not totally shifting to a nocturnal schedule, but you are getting the time of day when your clock maximally promotes sleep to be outside your work hours. You can do this by blasting yourself with light in phase delay portion of your body’s daily rhythms, which for a person who’s still pretty adjusted to the day schedule is going to be in the afternoon and evening. Note that this is where we start to conflict a bit with the homeostat-targeting interventions: If you’re keeping yourself in a super bright environment in the hours before your shift, you’re probably not sleeping the whole time you’re at it.

Figure 4 from Burgess et al. 2002. This schedule is a compromise between work days and off days where the schedule on off days is still very late. Shaded rectangles are sleep windows, the large white rectangles represent five night shifts (11pm – 7am), and the L symbols are recommended times for light exposure. The black triangle markers show the timing of the core body temperature minimum, or the point at which your body sends its strongest signal for sleep.

Way #2, or advancing the clock, does not come with the same homeostat conflict. To advance the clock, a person still relatively well-adjusted to a day schedule would want to avoid evening/afternoon light and get tons of it in the morning. A “sleep after 1:00 pm” intervention in which people were also dosed with bright light in the latter part of their shift saw a 3 hour shift in the timing of the circadian rhythm biomarker, dim light melatonin onset (DLMO). In other words, you can target the homeostat right before a shift and promote an earlier phase shift at the same time.

Figure 1B from Chinoy et al. Black bars are scheduled sleep periods, gray bars are periods of dim light exposure, shaded bars are work shift hours, and white rectangles are periods of enhanced light during the night shift. By sticking the brightest light in the phase advance period for these people, their clocks should shift to better adjust them to sleeping from 1:00 pm to ~8 or 9:00 pm.

There’s evidence that both strategies can improve upon a baseline of undirected, “do what you want” advice to shift workers. Advancing the clock plays nicely with “sleep before shift” strategies, but you could also take a pre-shift nap, while mostly delaying yourself in the lead up to it. You could also try splitting your sleep—sleeping right after your shift, and then again right beforehand, and using your non-sleep time to steer your clock in one direction or another (though depending on what your personal time zone is, this may be a bit difficult—those hours might be times when you’re more or less insensitive to light).

So how do we begin to choose a strategy to recommend? Well, there’s one missing dimension to all the research touched on so far that we haven’t discussed yet.

Non-shift workers vs. shift workers

All of the shift working studies cited above looked at non-shift workers who were brought into the lab and put on simulated shift work protocols. Typically, being a shift worker was an exclusion criteria for the study: No real shift workers allowed.

There’s a very good reason for this, which is that shift workers have wonky circadian rhythms. You bring shift workers into a lab and look at their dim light melatonin onset timings, and you can see coverage over almost all the 24-hour clock. This means that you wouldn’t expect a nice clean scientific result to come out of putting them all on the same schedule: What’s good for someone would almost certainly be terrible for another. Focusing only on non-night shift workers (who are, it should be said, a good model for “just starting out on the night shift workers”) means you’re able to better parse a signal from the noise.

But it also means that you miss out on a very important piece of information: Namely, that only a tiny fraction of shift workers phase advance themselves in the real world. Many of them don’t follow particularly great strategies, but the ones who are better adapted tend to be very delayed.

From Gamble et al. 2011. Gray shaded areas are nigh shift work schedules, red shaded areas are typical sleep times. From a large number of responses, the Night Stay, Nap Proxy, No Sleep, Switch Sleeper, and Incomplete Shifter strategies were identified as categories. No Sleep and Nap Proxy shift workers tended to be worse adapted, while Incomplete Shifters and Switch Sleepers were better adapted.

This result comes from work in night shift nurses that looked at the different strategies that real nurses employ. In that research, the “most adapted” nurses were the ones who basically did this compromise phase position strategy, where they were very late types on their off days. Nurses who stayed up all night before a shift or napped during the day on their off days tended to be worse adapted— worse mood, increased cardiovascular risk, you name it. Counterintuitively, the least adapted nurses also tended to be older and more experienced on the job.

When you step back and think about shift work in a vacuum, the truly best strategy from a health perspective would probably be for shift workers to shift their lives entirely to align with night work, sleeping during the day even on the days they have off. In that sense, it would be like living in the United States but pretending you worked the same hours as a person living in Tokyo. With good enough blackout curtains and strong enough willpower to ignore the FOMO of diurnal life, you truly could fully adapt to a night-living lifestyle.

A tiny fraction of real shift workers do this. But most don’t, and the vast majority want to sleep at night during the days they’re not working. The better adapted nurses in the Vanderbilt study achieved this by being pretty extreme night owls on their days off. The poorly adapted nurses, the older ones who tended to stay up all night or nap on off days— they might be the ones to benefit most from a phase advancing schedule, which appears to have worse discoverability (nobody really does it in the real world) than the delaying schedules. In other words, if one direction isn’t working—as it appears not to for the ill-adapted shift workers—try going the other way.

Time now for my caveat that this is all, once again, pretty complicated. You can be an extreme night owl on your off days right up until the moment you have to work a 7am to 7pm shift. Your actions during your off days and off hours are constantly shifting your circadian profile, so that the thing that works for you one week might not work for you the next week. None of these studies could look at DLMO changing day-by-day in the real world, because none of them had the ability to track DLMO cheaply and in real-time. What do you want to prioritize—safety on the commute? Safety during shift? Ability to sleep well and feel good? Putting one of these above the other can give you a different answer. It’s a lot.

Enough already! What should I do?

Listen, if there was a one-size-fits-all easy solution to all of this, we wouldn’t have made an app for it. I would just have emailed everyone this blog post and done that thing where you brush your hands together in the international sign of “all done here.”

Here’s one rule-of-thumb, though: If you’re adjusted to a day schedule, and you’ve got a one-off night shift tonight before going back to the day schedule, you’re not going to be able to meaningfully shift your body’s circadian clock in the next 8 hours. You’re going to want to bank as much sleep as you can in the hours leading up to it and be aware of when your peak fatigue hours are going to occur. Our app can help you with that.

For everyone else, this is where our app comes in. Shift builds on this history of research to design plans unique to your body clock. You can choose which ones to try, and give feedback on the ones you like and don’t like. Want to help us move the needle on getting shift workers to a healthier place? Reach out for early access.

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Circadian science Lighting Sleeping troubles

Official Company Stance on Permanent DST

NOOOOOO!!!!

Noooooooooooooooooooooooo!!!!!!















(To hear our actual stance on permanent DST, check out this blog post. Short version: we love getting rid of the seasonal time change, as long as we end up on permanent standard time, not permanent DST.)

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Circadian science Lighting Shift Work

Book of the Month (January)

The Circadian Code by Satchin Panda

In keeping with the theme of new beginnings, this January we introduced a book of the month. Join us as we work our way through books that highlight the importance of circadian and sleep health 😴 Up first: The Circadian Code by Satchin Panda of the Salk Institute for Biological Studies.

We’ve been fans of Dr. Panda’s work for a long time! For more on his research, check out our blog post on time-restricted eating.

Chapter 3: “A healthy lifestyle includes what and when you eat, when and how much you sleep, and when and how often you move. By focusing on the when, you are harnessing the power of your circadian code, which can compensate for those times when you make less than exemplary choices. Better still, by living in alignment with this internal rhythm, you reap even greater benefits that come along with making good lifestyle choices.”

This excerpt is definitely speaking our language. We love anything that sings the praises of a circadian-aware life. At Arcascope, we believe that everyone deserves to reap the benefits of living in alignment with their internal rhythm. Our app, Shift, will make it easy to do just that.

Chapter 4: “A short nap during the day is one way to repay your sleep debt. The only times when napping really works against you are when you are jet-lagged, if you are a true shift worker and you want to sleep at night, or if you are really trying to move your bedtime to earlier in the evening. In these instances, it’s better to build up your propensity to sleep at night, and then reset your clock the next morning. “

A great section to highlight. There’s a ton of focus on enough hours of sleep per day, and not enough focus on when those hours of sleep happen. Naps are great, but if you’re a shift worker or trying to shift your sleep earlier, a nap at the wrong time can throw you off course.

Chapter 4: “Small lighting changes can have a huge impact. I’m not suggesting we spend the evenings in a dark room until we go to bed There are many techniques and products that can help reduce our exposure to blue light. For instance, in the evenings, shut off overhead lights and use table lamps instead.”

Yep, this line resonated with us. Because it’s not all or nothing when it comes to living a healthier #circadian life. You don’t need perfect darkness in your evening home environment or constant sunlight during the day. Moderate, realistic changes to your daily routine can be enough to help put your body’s clock back on the right track, and our recommendations are designed with this principle in mind. We bet you’ll be surprised by how easy it is to live in alignment with your body’s clock and especially how many benefits come from doing so.

Chapter 5: ” Our brain clocks are most sensitive to light, but the clocks in our gut, liver, heart, and kidneys respond directly to food. Therefore, just like the first sight of morning light resets the brain clock and tells it that it’s morning, the first bite or first sip of coffee of the day tells the clocks in our gut, liver, heart, and kidneys to begin the day. If we change our routine from day to day, our clocks get confused.”

It makes sense to think that our bodies might be more prepared to handle food at some times (like when we’re awake), rather than others (like when we’re supposed to be asleep). And the same way light at night confuses and disrupts the central clock in our brain, so too could food around the clock confuse and disrupt the peripheral oscillators in our organs.

Chapter 8: “If you wear blue-light-filtering glasses, then you don’t have to change the light bulbs in your home or find apps for your laptop or television.”

Another reminder of how simple changes to our daily routine, like wearing glasses at certain parts of the day, can help out our body’s clock.

Categories
Circadian science Lighting

Tis the Season: Seasonal Effects and Circadian Rhythms

As today is the shortest day of the year in the Northern Hemisphere (and the longest in the Southern Hemisphere), it seems appropriate to talk about how the seasons change our bodies’ rhythms. Many things change with the seasons, but the main seasonal variation that I will consider here is the variation in day length. 

The seasonal variation in light duration is a big change experienced as you move away from the equator. 

A contour plot of the number of hours of daylight as a function of latitude and day of the year. (Courtesy Wikipedia)

Growing up in Texas, I didn’t really appreciate this variation, but during my graduate school years in Michigan, I experienced it first hand. Personally, the short winter days were a bigger adjustment than the temperature. It felt like anytime I had to drive it was dark and snowing. 

Seasonal changes are known to induce all kinds of physiological changes in our bodies. These include changes in the immune system (Nelson and Weil 2015), physical activity (Shephard and Aoyagi 2009), weight gain (Baranowski et al. 2014), and hormonal changes (Tendler et al., n.d.). Even hair growth changes with the seasons (Shephard and Aoyagi 2009; Randall and Ebling 1991). 

One of the more surprising seasonal cycles is that human reproduction varies seasonally. So called “birth pulses” have been found to occur seasonally within the United States (Huber et al. 2004). Suicide numbers peak in the late spring/early summer. This is also the time of year associated with peaks in aggression and violent acts such as homicide and mass shootings (Geoffroy and Amad 2016). 

Figure taken from Stevenson et al “Disrupted seasonal biology impacts health, food security and ecosystems”, Proceedings of the Royal Society B, Oct 2015. (a) Show the suicide rates in Japan (b) Minor assaults in England and Wales.

Historically, battles and other aggressive behaviors have been shown to peak in this season as well. With all of these seasonal variations, it is natural to ask how the body keeps track of the seasons. Also, what does this have to do with circadian rhythms and the mission at Arcascope? 

First, a bit of background. Our daily rhythms are driven by biological clocks found throughout the body. The most important of these clocks is the central circadian clock located in a region of the hypothalamus called the suprachiasmatic nucleus (SCN). The central clock coordinates and synchronizes the other clocks found throughout the body, and—importantly —the central clock receives light information directly from the eyes. These light signals are the primary mechanism by which our bodies’ internal clocks stay aligned to the outside world. 

It turns out that this central clock is also responsible for maintaining the body’s record of seasonal information (Hannay, Forger, and Booth 2020; Coomans, Ramkisoensing, and Meijer 2015). It is both a daily clock and an annual calendar. This means the core clock has to somehow maintain a longer-term memory of the light it has seen over the past weeks and months. After all, you wouldn’t want to switch into winter mode just because of an especially cloudy afternoon in July. 

The daily 24-hour clock can be found ticking inside each of the individual neurons in the SCN. By averaging across these neurons at the population level (there are around ten thousand of them), you can arrive at a consensus daily time. The seasonal clock seems to work differently: important parts of the seasonal calendar are stored at the population level. This means each individual cell doesn’t know if it is July or January, but if you look at the whole population you can see seasonal changes. In other words, while the daily clock is stored by the consensus or average of the individual clocks, the seasonal information is encoded in the spatial patterns of the clock neurons. 

Interestingly, the seasonal patterns can also feed back on the daily clock and change how it operates (Pittendrigh and Daan 1976). For example, keeping lab animals in summer or winter conditions is known to cause lasting changes to their circadian clocks (the intrinsic period of the clock). These changes can persist for months after they have been moved into a different lighting environment. Another example is that mammals kept in lighting conditions close to long summer days are less light-sensitive than those kept in winter conditions (Pittendrigh and Daan 1976; vanderLeest et al. 2009). One explanation is that the clock needs to be more sensitive to light in the winter for the obvious reason that less light is available (Hannay, Forger, and Booth 2020). 

In modern life, our light exposure patterns do not differ as widely across the seasons due to artificial lighting. In fact, the average light exposure is closer to a perpetual summer (Wehr 2001). This perpetual summer environment has been found to maintain a summer-like state in the melatonin cycle (Wehr 2001). This movement towards perpetual summer has likely suppressed seasonal cycles which are important to maintaining health (Wehr 2001; Stevenson et al. 2015). Mice kept in constant artificial light have been found to have bone deterioration, reduced skeletal muscle function, and disrupted immune function. In humans, it has been shown that natural lighting conditions (camping) leads to a lengthening of the biological night during the winter months (Stothard et al. 2017). 

It is clear that the seasonal variation is important to our health and that modern artificial lighting is disrupting those cycles. On top of that, the long-term memory of light exposures means that these seasonal changes can also affect the operation of the daily clock. At Arcascope our core mathematical models are built to incorporate these seasonal variations— all as part of our goal of helping people maintain healthy daily and seasonal rhythms. 

Citations

Baranowski, Tom, Teresia O’Connor, Craig Johnston, Sheryl Hughes, Jennette Moreno, Tzu-An Chen, Lisa Meltzer, and Janice Baranowski. 2014. “School Year versus Summer Differences in Child Weight Gain: A Narrative Review.” Childhood Obesity  10 (1): 18–24.

Coomans, Claudia P., Ashna Ramkisoensing, and Johanna H. Meijer. 2015. “The Suprachiasmatic Nuclei as a Seasonal Clock.” Frontiers in Neuroendocrinology 37 (April): 29–42.

Geoffroy, Pierre Alexis, and Ali Amad. 2016. “Seasonal Influence on Mass Shootings.” American Journal of Public Health.

Hannay, Kevin M., Daniel B. Forger, and Victoria Booth. 2020. “Seasonality and Light Phase-Resetting in the Mammalian Circadian Rhythm.” Scientific Reports 10 (1): 19506.

Huber, S., M. Fieder, B. Wallner, G. Moser, and W. Arnold. 2004. “Brief Communication: Birth Month Influences Reproductive Performance in Contemporary Women.” Human Reproduction  19 (5): 1081–82.

Nelson, Randy, and Zachary Weil. 2015. “Faculty of 1000 Evaluation for Widespread Seasonal Gene Expression Reveals Annual Differences in Human Immunity and Physiology.” F1000 – Post-Publication Peer Review of the Biomedical Literature. https://doi.org/10.3410/f.725486269.793507034.

Pittendrigh, Colin S., and Serge Daan. 1976. “A Functional Analysis of Circadian Pacemakers in Nocturnal Rodents.” Journal of Comparative Physiology ? A. https://doi.org/10.1007/bf01417860.

Randall, V. A., and F. J. Ebling. 1991. “Seasonal Changes in Human Hair Growth.” The British Journal of Dermatology 124 (2): 146–51.

Shephard, Roy J., and Yukitoshi Aoyagi. 2009. “Seasonal Variations in Physical Activity and Implications for Human Health.” European Journal of Applied Physiology. https://doi.org/10.1007/s00421-009-1127-1.

Stevenson, T. J., M. E. Visser, W. Arnold, P. Barrett, S. Biello, A. Dawson, D. L. Denlinger, et al. 2015. “Disrupted Seasonal Biology Impacts Health, Food Security and Ecosystems.” Proceedings. Biological Sciences / The Royal Society 282 (1817): 20151453.

Stothard, Ellen R., Andrew W. McHill, Christopher M. Depner, Brian R. Birks, Thomas M. Moehlman, Hannah K. Ritchie, Jacob R. Guzzetti, et al. 2017. “Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend.” Current Biology: CB 27 (4): 508–13.

Tendler, Avichai, Alon Bar, Netta Mendelsohn-Cohen, Omer Karin, Yael Korem, Lior Maimon, Tomer Milo, et al. n.d. “Human Hormone Seasonality.” https://doi.org/10.1101/2020.02.13.947366.

vanderLeest, Henk Tjebbe, Jos H. T. Rohling, Stephan Michel, and Johanna H. Meijer. 2009. “Phase Shifting Capacity of the Circadian Pacemaker Determined by the SCN Neuronal Network Organization.” PloS One 4 (3): e4976.

Wehr, T. A. 2001. “Photoperiodism in Humans and Other Primates: Evidence and Implications.” Journal of Biological Rhythms 16 (4): 348–64.

Categories
Circadian science Lighting

This Thanksgiving, Get Outdoors

Here’s a fun fact: You probably get way less light exposure during a normal work day than you would if you were out camping.

“Sure,” you say. “That’s no surprise. At home, I have walls around me to block the sun. If I’m camping, I presumably have fewer walls.”

“You don’t understand,” I say, leaning in. “You get way, way less light exposure.”

I’m basing this off a famous circadian experiment from Ken Wright’s group at the University of Colorado Boulder, in which they compared the light people get in modern electrical lighting environments with the natural light they get while camping. 

It’s not a 1x or 2x difference when you go from modern light exposure to camping light exposure. It’s a 13x difference

Thirteen times more light exposure during the day! And this in the winter! It’s nuts. 

Imagine turning your current daily light exposure down by a factor of 13, or to 8% of its current brightness. Two things would probably be true. First, it would be hard to see, so you’d bump into things. Second, and most important from a circadian perspective, it would be hard for your brain to tell the difference between day and night. 

After all, the signal telling your brain that it’s day (light) would be just a tiny fraction of what it was before. It’d be like turning a faucet down to just a thin drizzle. You can tell it’s on if you look for it, but it’s an easy thing to miss. 

In a sense, we’ve already done this with the shift from natural lighting to indoor, artificial lighting. We’ve given up the firehose of light that is sunlight exposure in favor of a much muted signal from our indoor lights and devices. 

If you look at the lighting figure above from Stothard et al., it’s ridiculously easy to see where day starts and stops in the camping conditions (black line). But the picture is muddled for modern electric light (gray line): Day seems to start fairly clearly, but where does it end? There’s this blunted peak in light exposure during the day, and a long, ambiguous tail of light exposure stretching out into the night hours. There’s not really a clear day/night divide. 

This matters for our health. There’s a notion in circadian rhythms science of your circadian “amplitude.” Roughly, you can think of amplitude as a measure of how confident your body’s clock is about the time it thinks it is. Give your circadian clock a clear day/night signal, and this will boost the amplitude. Keep it on a constantly changing, dim-light-round-the-clock kind of schedule and the amplitude goes down. 

In other blog posts, I’ve talked about your phase response curve, which tells you which direction (earlier, later) light will push you when you get exposed to it. But you can also think of the amplitude response curve, which tells you whether your amplitude will go up or down if you get light at a certain time. Generally speaking, the amplitude response curves in our models tell you to go outside right smack in the middle of the day if you want to boost your amplitude as efficiently as possible. 

So this Thanksgiving, get some outdoor light. Sure, yeah, get some exercise while you’re out there if you want. But simply being outside and in the light is a good thing: It’s building stronger, more robust rhythms in your brain. And if you happen to fall asleep hard after eating a big meal– well, part of it might be that your circadian clock’s a little more confident that day is over and it’s time to snooze. 

…part of it. 

Categories
Circadian science Lighting

inTRO to ipRGCs ( Intrinsically Photosensitive Retinal Ganglion Cells)

Rods, cones, and…ipRGCs?

For almost a century and a half, it was thought that the mammalian retina had just two types of photoreceptors: rods and cones. That assumption was not proven to be false until studies in the late 1990s proved the existence of a third kind of mammalian photoreceptor that differed greatly from rods and cones. These new types of photoreceptors were retinal ganglion cells (RGCs) that were intrinsically photosensitive (ipRGCs)— or in other words, naturally sensitive to light.

Though the official evidence to determine that ipRGCs actually existed did not come until much later, this third class of photoreceptor had already been hypothesized in 1927, nearly seven decades earlier, by a graduate student named Clyde Keeler. During one of his studies, he examined the behavior of mice that lacked nearly all rod and cone function as a result of severe retinal degradation, which left them functionally blind. Keeler noticed that despite the lack of rods and cones, the mice still had a very strong and significant pupillary constriction in response to light, and he determined that this response must have been the result of some third photoreceptor in the retina. The lack of concrete evidence for a whole new photoreceptor at the time resulted in this pupillary response being explained away by other scientists. However, in 1999, Russell Foster and his team would revisit Keeler’s work armed with a new host of tools.

Foster et al. worked with mice, much like Keeler did, but in their case, the mice being observed were genetically engineered to not have any rods or cones. Yet regardless of their missing rods and cones, the rats still displayed strong pupillary light reflexes and were even able to shift their circadian rhythms with shifting light exposure schedules. With these studies complete, the presence of a third photoreceptor was almost confirmed, but some still weren’t convinced because nobody had found another light-sensitive molecule (opsin) in the mammalian retina yet.  

The discovery of melanopsin in the photosensitive skin cells of frogs occurred in 1998, and in the following four years studies determined that the very same opsin was being expressed in a small percent of RGCs in both mouse and human retinas. This discovery allowed scientists to easily mark ipRGCs and confirm their existence, which finally put to rest the debate of whether or not there was a third class of photoreceptor.

So they exist, but what do they do?

IpRGCs differ greatly from rods and cones when it comes to how they work. Their main function in the body is to signal the intensity of ambient light levels (irradiance) to the brain. These signals are largely used for non-image-forming visual reflexes that are subconscious, such as pupillary constriction, neuroendocrine regulation, and synchronizing daily circadian physiological rhythms to environmental light. This means that the way ipRGCs respond to light by themselves is also quite different from rods and cones.

As mentioned before, these photoreceptors use melanopsin as their photopigment. and that makes them more responsive to light at around 480nm (blue light). In the graph below, you can see that this wavelength is significantly different from the best wavelengths for stimulating rods and cones (panel b).

From: https://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/melanopsin-expressing-intrinsically-photosensitive-retinal-ganglion-cells/

Although ipRGCs function as photoreceptors themselves, it was found that they additionally receive synaptic input from the circuits of rods and cones. This means that ipRGCs have both an intrinsic light response coming from melanopsin and an extrinsic one that is mediated by synaptic input from rods and cones. The light response caused by melanopsin is markedly different from that of rods and cones: ipRGCs have both an intrinsic and sluggish light response as well as an extrinsic, rod/cone driven, rapid photoresponse. There is an ongoing debate about the relative significance of this extrinsic synaptic input and the role rods and cones play in determining our circadian rhythms.

A recent case study:

In a recent research article by Mouland et al., their team assessed whether the effective light intensity registered by melanopsin (blue light ~480nm) was a more important determinant of circadian impacts than that of cones under realistic contrast scenarios. The ability to determine melanopsin’s contribution to circadian light responses comes from the evolution of a color science technique which is referred to with multiple names, such as receptor silent substitution or metamerism in colorimetry. Metamerism occurs when two colors appear to match under a specific lighting condition but have different underlying spectra. 

From: https://en.wikipedia.org/wiki/Metamerism_(color)

This technique allows for the stimulation of specific photoreceptor classes, like ipRGCs. Mouland and colleagues quantified the circadian impacts of different photoreceptors by recording electrophysiological activity from the suprachiasmatic nucleus (SCN) of anaesthetised mice while they were presented with movies. The movies were either high or low contrast and had varying irradiances specialized for the distinct photoreceptor classes. 

During the experiment, the energy response recorded from the SCN closely tracked with melanopsin-driven signaling across all conditions. In general, steps in melanopic irradiance were determined to be the most significant factor accounting for light-induced changes in SCN activity. The only cone-directed lighting patterns with significant impacts on SCN activity were low contrast movie conditions. Basically, this study suggests that cones do have an impact on the circadian signal going to the SCN in some conditions, but the influence of melanopsin on the circadian signal is far more consistent.

This blog post was written by Arcascope’s intern, Ali Abdalla. Thanks, Ali!

This post used Webvision as a major resource. Thanks to Dustin Graham and Kwoon Wong for the excellent review.

Categories
Circadian science Lighting Sleeping troubles

No, we shouldn’t make DST Permanent

I recently got some blackout curtains for my bedroom. This was pretty long overdue: about thirty feet from my bedroom window is a cheerfully bright, energy-efficient street lamp, which—while great when I’m taking the dog out for a nighttime stroll—is the photic equivalent of somebody standing in my azaleas and playing “Seventy-Six Trombones” while I’m trying to sleep.

I’ve definitely started sleeping longer since I’ve gotten them. But I’ve also noticed that they’ve made it so I need to be even more careful about my other sources of light at night. The reason? They don’t just block my light at night. They also block light in the morning.

I’m thinking about this because it’s almost the end of daylight saving time, and, once again, there’s talk of making it permanent. As a quick guide: Daylight saving time (DST) is the one where the clocks move forward (so it’s lighter at night), while standard time is the one where the clock moves back (darker at night). The “Sunshine Protection Act”, introduced by Florida senator Marco Rubio, encourages states to observe a permanent version of DST, with the argument being that lots of good things could come out of just chilling it with the time change.

Permanent daylight saving time means not having to change the clocks, and not having to experience that gnarly “lose an hour” in the Spring. It means no confusion about how many hours offset we are from the time in the U.K. and no struggling to remember if you should say EDT or EST when you’re trying to coordinate a Zoom meeting across time zones. As a programmer, I’m generally in favor of anything that makes the totally miserable experience of interacting with dates and times in code even marginally easier.

But it also means—and I’m talking about permanent daylight saving time here—lots and lots of dark in the mornings.

This is bad. It’s bad because light at night is fundamentally different from light in the mornings, because our bodies are fundamentally different at night than they are in the mornings.

Note: This picture does not apply if you’re a shift worker, a recent traveler, or otherwise circadianly weird.

Light in the morning does a couple things, but one of the most important ones is that it “advances” our circadian rhythms. It tells our internal clock that night is over and it’s time to get a move on. It makes it easier to fall asleep at night.

And if you get a lot of light in the morning, it eventually advances you to the point where… it stops advancing you. You enter the part of your daily rhythm where light delays your clock. A sort-of “slow down, what’s the rush” period of your internal rhythm that starts in the mid-afternoon for most people and continues into the early morning.

More light in the morning: The permanent standard time solution.

And that slowdown period is the problem. Because while light in the morning is hitting you in the advance region, which you eventually get advanced out of, light at night is hitting you in the delay region, which is like a temporal sand trap. When you get light exposure in the delay region, your clock gets slowed down, which means you spend more time in the delay region. Which means you don’t feel tired as quickly, which means you get more light, which means you spend even more time in the delay region. It’s a feedback loop that spins out of control. It might be the reason that night owls exist.

Permanent DST (gasps in circadian horror)

So if we adopt permanent DST, we’re adopting a schedule where we get more light during the hours most people call night, and much less light in the hours we consider morning. We’re setting ourselves up to fall into the delay region sand trap: More light in the night, making us stay up later and get delayed, and far less light in the AM hours to counteract it.

This is what tanked permanent DST the first time we tried it. I’m not sure why this doesn’t always get brought up as the very first point against permanent DST, but we’ve totally done it before. In 1973, anywhere from 57-73% of people supported staying on DST during the winter. So they did it, in January of 1974. By the time February and March rolled around, only 19-30% of people still thought it was a good idea, while 43% said it was actively bad.

What changed? People experienced what happens to your body when you have to kick off your day in the dark of night. They drove to work and caught the bus to school, while the sun waited to rise until 8:00 am. They didn’t like it, and rolled the decision back before the next winter came around.

You might say, “well, time is a fake idea. Who says you have to start your day before 8:00 am?” This is a fair point. We could, societally, shift the normal times we do things to match whatever schedule we wanted. In China, where the entire country is on the same time zone, places like Kashgar (in the far west) have shifted their normal operating hours to reflect the fact that the sun might not come up until 10 am.

But it’s a lot tougher to change social standards of when school and work “should” start in every town in the country than it is to pass a bill changing the time that appears on your phone. Which is why we shouldn’t do it: Permanent DST will put us on schedule where our traditional social standards for when things should happen are at odds with our biology, sabotaging our sleep and circadian health.


If we want to stop the whiplash of changing the clocks twice a year, why not do permanent standard time? I’m in favor of this. It reduces confusion the same way permanent DST does, but without the corresponding damage to our internal rhythms. Sure, it might mean that 9:00 pm is dark, even in the summer. But darkness at the right times is a healthy thing. And from a safety perspective, there are lots of street lamps and other sources of light at night these days that are very good at their jobs.

Which brings it back to me and my blinds: I’ve needed to be more careful about my other sources of light at night lately because my blackout curtains mean I don’t get woken up by the sun. That’s not a big problem: I can wake up in the dark and yank them open myself, like one of the townsfolk in the first song in Beauty and the Beast.

But if I get too much light at night from non-streetlamp sources, like watching Succession on my computer or looking at Succession memes on my phone, my ability to wake up in the dark in the morning is going to be less reliable, jeopardizing my exposure to that vital morning light. And I’m lucky that there’s even morning light to get: with permanent DST, I could be hopping on my first calls of the day while the sky is still black outside.

My point is that social pressures already make it hard for us to get the darkness we need at night (let’s face it, screens are fun) and the light we need in the morning. We shouldn’t make it harder for ourselves with a change to a system that’s already failed once. Permanent daylight saving time is a no-go. Permanent standard time? Call me.