The problem with generic timing advice
You've probably heard that light exposure resets your circadian rhythm. "Get sunlight in the morning to shift your rhythm earlier." That's not wrong, but it's incomplete. The same sunlight that can shift your rhythm earlier in one situation can shift it later in another situation. The timing is the thing that matters.
This is where phase-response curves come in.
What a phase-response curve is
A phase-response curve (PRC) is basically a map that shows what happens to your circadian rhythm when you expose it to a stimulus (usually light) at different times of day.
The curve shows two things:
- Amplitude: how much your rhythm shifts
- Direction: whether it shifts earlier (phase advance) or later (phase delay)
The key insight: the direction of the shift depends on when the stimulus happens. Light exposure in the morning shifts your rhythm earlier (advances it). Light exposure in the evening shifts it later (delays it).
Why this matters for shift workers
Imagine you just finished a night-shift block and you're trying to reset to days. If you get bright light exposure in the morning (when your body thinks it's evening because you've been awake all night), you're actually delaying your rhythm further away from daytime. If you instead get light exposure in the evening after sleeping, you can phase advance your rhythm back toward a normal schedule.
Generic advice like "get morning sunlight" doesn't account for the fact that your internal clock might be running 12 hours different from clock time. The wrong light exposure at the wrong time can make the adjustment harder, not easier.
The research on light and circadian shifting
Studies on phase-response curves show that bright light (around 10,000 lux, much brighter than indoor lighting) can shift your rhythm by about 1 to 2 hours per exposure, depending on the timing relative to your current circadian phase.
This is actually useful. It means that a strategic light exposure can shift your rhythm in the right direction, but a bad timing can shift it in the wrong direction or have no effect at all.
The limits of what we know
Phase-response curves are well-documented for bright light exposure, but less is known about the curves for other cues (exercise, food timing, temperature). The research suggests these other cues matter, but the timing effects are less dramatic and less well-characterized than light.
Also, individual differences matter. Your specific PRC might be slightly different from the population average, which is why some people find timing advice works great while others find it doesn't help at all.
How to use this
If you're trying to shift your rhythm on a night shift schedule:
- If you want to advance your rhythm (move toward a daytime schedule), get bright light exposure after your sleep, not before it.
- If you want to maintain your current rhythm (stay on nights), avoid bright light when you wake, and get light exposure just before bed if possible.
- The stronger the light, the bigger the potential shift, but even moderate light exposure has an effect.
ShiftFit can't control your light exposure, but training timing that matches your actual sleep cycle is one part of a bigger picture of supporting your body's attempts to adapt to a non-standard schedule.