Over and over again, I’ve heard shift workers say something like the following:
“I have no rhythms.”
“It’s just a constant experience of bleh.”
“I get off shift, and more than tired, I just feel… flat.”
“Ah,” I think when I hear this. “The amplitudes of their circadian rhythms have been squashed.” To steal an analogy from an older blog post, it’s like they’re rocking back and forth just a little bit on a swing at the park, instead of getting some nice height and momentum. It’s like they’re a yo-yo that’s only making it a fraction of the way back up to your hand. Or experiencing one of those dud bounces on a trampoline, where you only go up a few inches while the friend next to you goes flying.
There are lots of ways of talking about this idea of lost amplitude.
You can think of the amplitude of an average daily activity profile, where you visualize a person’s normal day by taking the average of what they do over multiple days. For somebody with an irregular schedule, the amplitude (or maximum height) of this profile is going to be lower than for somebody with a super regular schedule: An irregular person’s periods of high activity will get canceled out by periods of low activity, whereas somebody who’s super regular will have all their high activity happening at roughly the same time.
You can also think of amplitude in terms of the brain’s suprachiasmatic nucleus, or SCN, where the core timekeeping of your body’s daily rhythms occurs. If all the neurons in the SCN are like “Yes! It’s daytime!” they’ll send a clearer, stronger signal to the rest of your body, whereas if half of them are like “It’s day!” and the other half are like “No, it’s night!” the signal will be… not so clear. Kind of muddled, really. A dud bounce.
I was thinking about circadian amplitude as I made my way through this recent paper from Zhang et al. in eBioMedicine. In this paper, the authors track the temperature and activity patterns of both day and night shift healthcare workers, and look at the ways in which they differ from one another.
For instance, the probability of a day worker resting at different times of the day in their dataset looked a little something like this:
While the same plot for night shift workers looked like this:
The black dashed average line for night shift workers maxes out around 0.6, while the same line for day workers hits ~0.9. In other words, we see a lower amplitude of the probability of resting (and a much more chaotic picture overall of night shift activity overall).
This is that “average daily activity profile” I mentioned above. But the authors also looked at patterns in chest temperature over the course of the day, finding 24-hour rhythms in 70% of the day shift workers and only 48% of the night shift workers.
What’s that mean? Well, it could reflect the fact that temperature and activity are correlated—you move a bunch, your temperature goes up—so the story we see in activity could simply be making itself known through temperature as well. But it could also be capturing a bit of the SCN discord I brought up earlier. Your body’s clock contributes its own 24-hr pattern to your daily temperature profile, so a flatter signal coming from your brain could make for a flatter body temperature pattern too.
I’ve written elsewhere about how more amplitude—or a clearer difference between night and day—seems to be linked to lots of good things, like lower cardiovascular disease. One nice thing about amplitude? It’s not fixed over your lifetime: it can change, dynamically, with your actions. Our app Shift isn’t just aimed at shifting the time of your body’s clock: we also want to help you to boost amplitude as well. Want to give it a shot, for yourself or your employees? Reach out at email@example.com.
Thanks to the authors of Digital circadian and sleep health in individual hospital shift workers: A cross sectional telemonitoring study for an enjoyable read!