Short answer
A phase response curve (PRC) is a graph showing how much a stimulus – usually light – shifts the body clock, depending on when it is given. The light PRC has a simple shape: light in the hours before your temperature minimum delays the clock, light after it advances the clock, and light in the middle of the day does little. It is the rulebook behind jet lag light timing.
Simple explanation
Imagine pushing a child on a swing. Push at the right moment and the swing goes higher; push at the wrong moment and you fight against it. Light works on your body clock the same way. The phase response curve is simply the map of “which moment does what” – it tells you when a burst of light will move your clock earlier, when it will move it later, and when it will barely register.
The key reference point is your core body temperature minimum, or CBTmin – the coldest, sleepiest point of your night, usually a couple of hours before you normally wake. Everything on the curve is measured relative to it. Light before CBTmin delays your clock (pushes everything later). Light after CBTmin advances it (pulls everything earlier).
Deep dive
Researchers build a PRC by giving a controlled light pulse at different internal clock times in different people, then measuring how far each person’s clock moved and in which direction. Human light PRCs were established by work such as Minors, Waterhouse and Wehr (1991) and, in careful laboratory conditions, Khalsa and colleagues (2003). The resulting curve has a characteristic shape:
- Delay region – light in the evening and first part of the night (before CBTmin) shifts the clock later. Delays are largest in the hours right before CBTmin.
- Crossover – near CBTmin the effect flips from delay to advance.
- Advance region – light in the late night and morning (after CBTmin) shifts the clock earlier.
- Dead zone – through the middle of the day, light produces little phase shift, because the clock is relatively insensitive then.
Two important refinements. First, the biggest shifts sit close to CBTmin, so small errors in timing near that point can flip a helpful advance into an unhelpful delay – a reason to trust a calculated plan over intuition. Second, CBTmin itself moves as you adjust: each day of successful shifting drags it earlier (for an advance) or later (for a delay) by roughly an hour, so the “seek light” and “avoid light” windows creep along day by day rather than staying fixed.
Melatonin has its own PRC that is roughly the mirror image of the light curve – evening melatonin advances, morning melatonin delays. Because the two curves are opposed, a good plan can combine light and (with medical guidance) timed melatonin so both push the same way. Eastman and Burgess (2009) is a well-known applied review turning these curves into practical pre-flight and post-arrival schedules.
Illustration (planned graphic)
The canonical PRC plot rendered as inline SVG: the horizontal axis is clock time relative to CBTmin (marked at zero), the vertical axis is the size and direction of the phase shift (advances above the line, delays below). The curve dips into delays before CBTmin, crosses zero at CBTmin, rises into advances after it, and flattens through the midday dead zone. A faint mirror-image curve can show the melatonin PRC for contrast. This is a genuine scientific plot, so it must be data-accurate SVG, never a decorative or AI image.
What it means for travel
The PRC is why jet lag advice splits so cleanly by direction. Flying east means you must advance your clock, so you want light in the advance region (local morning) and must avoid light in the delay region (late evening). Flying west means you must delay your clock, so you seek evening light and avoid early-morning light. The eastward vs. westward tool explains that split, and the light planner applies the curve to your exact route, moving the windows a little each day as your CBTmin migrates.
A common mistake the PRC explains: after a big eastward trip, bright light too early in the local morning can land before your still-shifted CBTmin and delay you instead of advancing you – the opposite of what you want. Following a plan that accounts for where your CBTmin actually is avoids this trap.
Common questions
What is CBTmin and why does the PRC revolve around it?
CBTmin is your core body temperature minimum, the coldest point of your night, roughly two to three hours before habitual wake. The light PRC is defined relative to it because that point marks the switch between the clock’s delay and advance regions.
Why is there a “dead zone”?
Through the middle of the biological day the clock is relatively insensitive to light, so light then produces little phase shift. The strong effects occur around the night and the transitions near CBTmin.
Does melatonin have a phase response curve too?
Yes, and it is roughly the opposite of the light curve: evening melatonin tends to advance the clock, morning melatonin to delay it. Any use of melatonin is a medical decision – consult a doctor or pharmacist.
Sources
- Khalsa SBS, Jewett ME, Cajochen C & Czeisler CA (2003). “A phase response curve to single bright light pulses in human subjects.” Journal of Physiology, 549(3).
- Minors DS, Waterhouse JM & Wehr A (1991). “A human phase-response curve to light.” Neuroscience Letters, 133(1).
- Lewy AJ, Ahmed S, Jackson JML & Sack RL (1992). “Melatonin shifts human circadian rhythms according to a phase-response curve.” Chronobiology International, 9(5).
- Eastman CI & Burgess HJ (2009). “How to travel the world without jet lag.” Sleep Medicine Clinics, 4(2) – applied light/melatonin PRC scheduling.
The shape of the light and melatonin PRCs and their use for travel are drawn directly from the peer-reviewed sources above; jetlag.info’s tools implement this science as a transparent, non-medical planning heuristic.