The Right Light

I recently went on a trip to California.

Coming from the East Coast, this was a three-hour shift west. To adjust to California time, I needed to delay my circadian clock. One catch, though: my flight out was extremely early in the morning. That meant, like it or not, I was going to advance myself as I set out on the journey.

Let’s back up a little. We talk about directions your internal circadian clock can shift as advances or delays. Think of advancing as hustling your clock along, making your circadian rhythms more like those of people in time zones east of you. Delaying, on the other hand, is like a temporary slowdown for your clock, making your rhythms more like people living to your west. Light at different times of the day advances or delays you, depending on your clock’s state when you’re exposed to it. 

For most people, light in the morning is going to advance them and light at night is going to delay them. Here’s a very hand-wavey, anthropomorph-y way to think of it. If you’re getting light early in the morning, the time-keeping neurons in your brain might think, “whoops, the sun’s already up, earlier than I was expecting it. Time to get a move on.” Meanwhile, if you’re getting light late at night, these same neurons might go, “What? Light still?! I thought the day was over, but clearly I was wrong. Time to drag my non-existent neuronal feet.”

But how early is early in the morning, and how late is late at night?

In other words, where does the changeover from delaying to advancing occur? And how different is early morning from mid-morning to late morning, and what about the same question, but for night? 

Circadian researchers encode the answers to all of these questions in something called a phase response curve (PRC). A phase response curve summarizes the amount of phase shift that occurs in response to a zeitgeber, or internal time-shifting signal, at different internal times. (Light’s the most important zeitgeber, but other zeitgebers include exercise and melatonin). PRCs are often defined relative to a circadian marker, like dim light melatonin onset (DLMO), urinary aMT6s acrophase, or core body temperature minimum (CBTMin). 

The shape of a PRC will depend on the zeitgeber– how long and bright the light is, for example. If you’ve got a very long pulse of light, odds are good that you’ll hit both the phase advance and delay regions during that interval for some stimulus application times, which will be less likely to happen for 1 hour pulses. And with brighter light, you’d expect bigger effect sizes. 

A PRC for 6.7 hours of bright light might look like this:

A PRC for 1 hour of bright light, on the other hand, could look like this: 

In my case, I theoretically wanted light exposure in the delay region, to move me to California time. But I ended up getting it near the start of my advance region, because I had to wake up earlier than usual to make my flight. Or, in picture terms: 

I say theoretically because (time for a dramatic plot twist) I went to California fully determined to stay on my East Coast schedule while I was there. It was a short visit, and I didn’t have any late evening plans while I was there. On my first night there, I was in bed and asleep by my normal (east coast) bedtime, even though the sun was still up.

The next morning, I woke up at around 3:30 am California time, or 6:30 am East Coast time. This is a little earlier than I normally wake up on the East Coast, but I wasn’t too surprised. After all, I’d advanced myself the day before, and I’d been careful to avoid delaying myself much in the evening. In other words, while my body had traveled west, the clock in my brain had traveled east.

The point of this whole story? There’s a lot more to phase shifting than just the time zones you’re crossing. It matters what times of the day you’re active and exposing yourself to zeitgebers. It matters how much you can control your lighting– I could have worn blue-blocking glasses to try to reduce how much I advanced, for instance, but I probably couldn’t have brought my own 10,000 lux lamp on the plane without incurring the ire of the other passengers. It also matters what your goals are: I was perfectly happy camped out in my hotel room in the early hours of the California morning, because I’d decided ahead of time that I wanted to shift as little as possible.

If this sounds complicated, that’s because it is. There are a lot of moving pieces when it comes to shifting your internal time with travel, and even more with the chronic jet lag of shift work. But this complexity is also what makes it such a juicy problem to work on: there’s a lot of cool opportunities for a lot of cool math.