A phase response curve (PRC) describes how much a signal, most often light, shifts your body clock, and in which direction, depending on when in your biological day it arrives. It's the reason the same 20 minutes of bright light can move your clock earlier in the morning and later at night.
The Shape of the Light PRC#
Plot the size of the shift against the time the light arrives and you get a curve with two halves:
- Light in your biological evening and the first half of the night moves the clock later (a phase delay). This is why bright screens and ceiling lights late at night make it harder to fall asleep the next evening.
- Light in your late night and morning moves the clock earlier (a phase advance). This is why morning daylight helps you wake up and feel sleepy on time.
- Around the middle of your biological day, light shifts the clock very little, though it still makes you more alert.
The crossover point sits close to your core body temperature minimum, which for most people falls a couple of hours before their usual wake time. Light before it delays you; light after it advances you. A careful lab study using single pulses of bright light mapped this curve in people (Khalsa et al., 2003, Journal of Physiology). The core body temperature guide explains the temperature low.
It Follows Your Body, Not the Clock on the Wall#
The curve is anchored to your internal time, not the time on your phone. For a night owl whose body runs two hours late, 7 AM light can still land in the delay zone, which is one reason very early light doesn't always help strong evening types, and why shifting in small steps works better. After a long flight, your body is still on home time, so the right window for light can fall at odd local hours.
Jet Lag: Where the PRC Earns Its Keep#
Flying east, your clock needs to move earlier, so you want light in your body's morning and darkness in its late night. Flying west, it needs to move later, so you want light in the evening. Get it backwards and you can push your clock the wrong way and make the jet lag last longer. Knowing when to avoid light is the half most travellers never hear; the avoid-light window explains it, and the free jet lag planner works out both windows for your trip.
The same logic applies twice a year when the clocks change. The daylight saving time planner times light in the right direction for spring and autumn.
Melatonin Has a Curve Too#
Melatonin taken as a supplement also shifts the clock, roughly opposite to light: taken in the afternoon or early evening it tends to move the clock earlier, and taken in the morning it tends to move it later (Burgess et al., 2010, Journal of Clinical Endocrinology & Metabolism). In that study, a low dose shifted the clock about as much as a higher one; timing mattered most. Talk to a doctor or pharmacist before using melatonin, especially for children or if you take other medication.
How big a shift light produces depends on how bright it is, how long it lasts, and how much light you've seen recently. Outdoor daylight is far brighter than indoor lighting, which is why a short time outside does more than a whole morning under ceiling lights.
Where ARC Fits#
ARC applies this curve when you travel: it notices your phone's time zone change (no location permission) and gives you a seek-light and an avoid-light window each day while your body catches up, about an hour a day. At home, its 20-minute morning light timer starts from your real sunrise.
Educational content, not medical advice. ARC is a wellness app, not a medical device.