Reading a phase response curve to see when light helps and when it hurts
A 2012 human light phase response curve puts the delay-to-advance switch about 9 hours after melatonin onset. Here is how to read it, with a worked example.
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Of the 39 volunteers enrolled in St Hilaire and colleagues’ 2012 light study, 34 made it into the analysis: 18 got one hour of bright light at an assigned circadian time, and 16 sat in dim light as controls. On the fitted curve, light that starts about 9 hours after dim light melatonin onset (DLMO) begins to produce advances, and light that starts between roughly 3 hours before DLMO and that 9-hour point produces delays.
What does a phase response curve measure?
A phase response curve is an input-output plot. The input is the circadian time at which a stimulus lands, and the output is how far the clock moved afterward, in hours, with delays negative and advances positive. Anyone who has pushed a child on a swing already understands the shape of it: the same push helps or hinders depending on where the swing is in its arc.
Two things make the curve unlike a dose-response chart. The x-axis is internal time instead of wall-clock time, so the same 7 a.m. light falls on a different part of the curve for different people. And the response is signed. A stimulus that is wonderful at one internal time is counterproductive at another, which is why generic advice to “get more light” is incomplete.
Where are the switch points on the one-hour curve?
St Hilaire et al. (Journal of Physiology, 2012) delivered one hour of about 8,000 lux while participants lived on constant-routine protocols, then measured the shift in DLMO across three subsequent circadian cycles. Control participants stayed under 3 lux. The authors fitted a two-harmonic curve to the shifts, and the fitted curve has two crossing points:
- The response changes from advances to delays about 3 hours before DLMO.
- It changes from delays back to advances about 9 hours after DLMO.
Delays were statistically significant for light that started from about 2 hours before DLMO to about 6 hours after it. Everything between the two crossing points leans toward delay. Everything outside leans toward advance.
The paper makes a small claim that overturns an older assumption: “there was no evidence of an extended ‘dead zone’, in which no phase shifts were generated.” A dead zone, a stretch of the cycle where light does nothing, appears in some circadian models. On this curve, after adjusting for the clock’s intrinsic drift, a one-hour pulse always moved something.
How do you translate DLMO into clock times?
Few people know their DLMO. For a worked example, assume one: DLMO at 21:00, a value picked for the arithmetic. The two crossing points then fall at 18:00 (3 hours before) and 06:00 (9 hours after).
| Light starts at | Position on the curve | Expected direction |
|---|---|---|
| 18:00 to 06:00 | Between the crossing points | Delay |
| 19:00 to 03:00 | Window where delays were significant | Delay, statistically significant |
| 06:00 to 18:00 | Outside the crossing points | Advance |
The three numbers to carry are the anchor (DLMO), the switch (minus 3 and plus 9 hours), and the size, which is where the curve gets interesting.
Why is evening light a stronger lever than morning light?
On the fitted curve the maximum delay is 1.75 hours and the maximum advance is 0.45 hours. A single hour of light is therefore worth about 3.9 times more at its best delaying point than at its best advancing point (1.75 divided by 0.45). The raw, unfitted data are less lopsided: a 2.02-hour delay against a 1.20-hour advance.
That is a counterintuitive result for anyone who has been told that the morning is the powerful time for light. The authors name a reason to hold the claim loosely. Constant-routine protocols involve sleep deprivation, which may suppress advances, so the advance side could be underestimated. The defensible reading is narrower than “evening beats morning.” It is that a single bright hour in the lab pushes a clock later more readily than earlier, and the size of the gap is uncertain.
How much does a long exposure add over a short one?
The 2012 curve was built to compare against Khalsa and colleagues’ 2003 curve, which used 6.7 hours of bright light. Fitted peak-to-trough amplitude was 2.20 hours for the one-hour pulse and 5.46 hours for the 6.7-hour exposure. One hour is about 15% of the exposure and returned about 40% of the amplitude.
Per hour of light, that is 2.2 hours of amplitude against 0.8 (5.46 divided by 6.7), a ratio of roughly 2.7 to 1. Two curves give only two points, so this hints at diminishing returns without proving a law: hours of light are not interchangeable, and the first hour appears to carry a disproportionate share.
What can this curve not tell you?
Several limits sit inside the paper itself.
- The one-hour fit explains modest variance (adjusted R-squared of 0.55), and the control group showed no discernible curve (adjusted R-squared of negative 0.03).
- The 1-hour and 6.7-hour curves have overlapping 95% confidence intervals except near the extremes of the longer curve.
- The participants were young adults, and the 6.7-hour study used an older group (19 to 44 years), so the comparison mixes age with duration.
- The pulse was 8,000 lux. Outdoor daylight is typically in the thousands to tens of thousands of lux, while ordinary indoor lighting is typically in the low hundreds, so most indoor hours fall well below the tested level.
- Real life has meals, exercise, and uneven sleep, all of which can nudge the clock without light.
The last point is the one engineers should care about most. A lab curve is an open-loop measurement of one input with everything else frozen. A person at home is a closed loop with several inputs and a drifting setpoint.
What does this change about picking a wake time?
Three practical readings survive the caveats. A wake time that puts bright outdoor light after the clock’s advance threshold works in the direction of an earlier clock. Bright light in the late evening works the other way and can be strong. And because the first hour appears to carry a disproportionate share, a short deliberate dose near the right point is a better bet than a long vague one. Morning light as an alertness tool rather than a clock tool is a separate question, taken up in the bright light therapy evidence. Travelers can set the light advice here beside the options ranked in the jet lag interventions breakdown.
Frequently asked questions
What is DLMO?
DLMO is dim light melatonin onset, the clock time at which melatonin in the blood or saliva begins to rise under dim light. In the 2012 study it served as the marker for where each person’s clock sat, and it was measured from plasma for most participants and from saliva for five.
Where was the 2012 curve published?
St Hilaire and colleagues published it in the Journal of Physiology, volume 590, issue 13, pages 3035 to 3045, in 2012, as a comparison with the 6.7-hour curve of Khalsa and colleagues.
Who were the participants?
They were young adults aged 18 to 30, studied under constant-routine laboratory conditions, with melatonin sampled before and after the light pulse.