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Light and the Phase Response Curve

Introduction

Light is the most powerful time cue available to the human circadian system. It does not merely wake you up, it physically shifts the timing of your biological clock, either earlier or later depending on when exposure occurs. Understanding this relationship is central to managing jet lag effectively.

The phase response curve is a tool that maps how the circadian clock responds to light across the 24-hour day. It explains why a bright walk at the right time can accelerate adaptation, and why light at the wrong time can make jet lag last days longer than it needs to.

Why Light Controls the Clock

The human eye contains two types of photoreceptors used for vision, rods (for low-light vision) and cones (for color and detail). But there is also a third class, discovered in the early 2000s: intrinsically photosensitive retinal ganglion cells, or ipRGCs. These cells do not contribute to what you see. Their sole function is to detect light levels and relay that information to the suprachiasmatic nucleus, the master circadian clock located in the brain’s hypothalamus (Berson et al., 2002; Hattar et al., 2002).

These circadian photoreceptors are most sensitive to short-wavelength blue light, around 460–480 nanometers (Thapan et al., 2001; Brainard et al., 2001). This is why blue-enriched light sources, including daylight, LED screens, and blue-spectrum lamps, have the strongest effect on the circadian system compared to warmer, red-shifted light of equivalent brightness.

The circadian response to light depends on three factors:

  • Wavelength: Blue light (~460 nm) produces the largest clock shifts and the greatest suppression of melatonin (the hormone that signals nighttime). Red light at the same brightness has minimal circadian effect.
  • Intensity: Brighter light produces larger shifts, up to a saturation point. Ordinary indoor lighting (~100–300 lux) has measurable but modest effects. Bright outdoor daylight or a dedicated light therapy lamp (2,500–10,000 lux) produces substantially larger responses (Zeitzer et al., 2000).
  • Duration: Longer exposures produce larger shifts, but with diminishing returns. Even brief high-intensity pulses can produce meaningful shifts.

The Phase Response Curve

The phase response curve maps how much and in which direction the circadian clock shifts in response to a light stimulus applied at different points in the 24-hour cycle. Two key features define it:

  1. The crossover point coincides with the core body temperature minimum, the lowest point of the body’s daily temperature cycle. This typically occurs approximately 2 hours before habitual wake time. For someone who normally wakes at 7:00 AM, the temperature minimum falls around 5:00 AM.
  2. Light before the temperature minimum causes delays (the clock shifts later). Light after the temperature minimum causes advances (the clock shifts earlier).

The largest shifts occur in the 3–6 hours on either side of the temperature minimum (Czeisler et al., 1989; Khalsa et al., 2003). A single session of appropriately timed bright light exposure can shift the clock by 1–3 hours. Repeated sessions over consecutive days accumulate these shifts.

Advancing vs Delaying the Clock

The direction of required shift depends on the direction of travel:

  • Eastward travel requires phase advances, the clock must move earlier to match the destination time zone. This is achieved by seeking light in the biological morning (after the temperature minimum) and avoiding light in the biological evening.
  • Westward travel requires phase delays, the clock must move later. Evening light exposure and morning light avoidance facilitate this shift.

Phase advances are physiologically harder than phase delays. The human circadian clock has a natural period slightly longer than 24 hours (approximately 24.2 hours on average), which means it drifts slightly toward delay each day. Eastward travel therefore demands effort against the clock’s natural tendency, which is why most travelers find eastward trips more disruptive.

Morning Light

Light exposure in the hours after the core body temperature minimum advances the circadian clock. In practical terms, this means light from approximately the last hour or two of sleep through several hours after waking produces the advance signal.

After eastward travel, the temperature minimum is anchored to the departure city’s time. If the traveler flew from New York to London (5 hours east), a temperature minimum that normally occurs at 5:00 AM New York time will initially fall at 10:00 AM London time. Light exposure during the late London morning, which falls after the traveler’s displaced temperature minimum, will progressively shift the clock earlier toward London time.

Optimal morning light for advancing the clock:

  • Bright outdoor daylight or a 10,000-lux light therapy lamp
  • Timing: starting after the displaced temperature minimum and continuing for 1–3 hours
  • Duration: 30–60 minutes of continuous exposure is effective; intermittent light is nearly as effective (see below)

Evening Light

Light in the hours before the temperature minimum, in the evening and early nighttime, delays the circadian clock. For most individuals, this means the biological evening window extends from roughly 8:00 PM through the early morning hours, anchored to the displaced clock rather than the destination clock.

After westward travel, the temperature minimum is displaced ahead of destination time. Evening light at the destination falls into the delay zone of the traveler’s displaced clock, which is exactly what is needed. Travelers flying from London to New York should seek bright evening light and social activity to facilitate the westward delay.

For eastbound travelers, bright evening light is counterproductive. It delays a clock that needs to advance, potentially extending jet lag duration.

Why Incorrect Timing Worsens Jet Lag

The phase response curve makes clear why light timing errors are consequential. Applying light in the delay zone when advances are needed, or vice versa, actively opposes adaptation. In severe cases, this can cause the clock to shift in the wrong direction entirely. For large eastward crossings (8 or more time zones), the clock may delay all the way around the 24-hour cycle rather than advancing the shorter distance, a process called antidromic re-entrainment, which can extend jet lag from 4–5 days to 8–10 days (Eastman et al., 2005).

A common scenario: after an 8-hour eastward flight landing in the morning at the destination, a traveler who stays in bright light throughout the local morning may be exposing their clock to light that falls in the delay zone (because their displaced temperature minimum is still in the late morning or midday local time). The practical solution is to estimate where the temperature minimum falls and avoid bright light in the hours before it.

Intermittent Light Exposure

Continuous bright light is not required for effective clock shifting. Research by Eastman and colleagues (2005) demonstrated that intermittent bright light, approximately 5,000 lux for 30 minutes alternating with dim conditions for 30 minutes, produces shifts nearly equivalent to continuous light exposure at the same intensity.

This finding is practical good news. Travelers who cannot sustain several hours of continuous outdoor or light-therapy exposure can use a structured intermittent approach, spending 30 minutes outside, then 30 minutes indoors in dim light, repeated through the target window, with comparable results.

Practical Implications

Applying the phase response curve to jet lag management requires three pieces of information:

  1. Your estimated core body temperature minimum at departure: approximately 2 hours before your habitual wake time at home.
  2. Where that minimum falls in destination time: shift your home temperature minimum by the number of time zones crossed.
  3. The direction of travel: advance needed (eastward) or delay needed (westward).

With these, the advance and delay windows can be identified and light exposure scheduled accordingly:

  • Seek light in the advance zone (after the temperature minimum, in the morning) for eastward adaptation.
  • Seek light in the delay zone (before the temperature minimum, in the evening) for westward adaptation.
  • Avoid bright light in the opposing zone, this is where timing errors cause problems.
  • Use blue-enriched light sources for maximum effect; use amber/red lighting or blue-blocking glasses when you need to avoid circadian stimulation.

Key Takeaways

  • Light is the primary external signal that shifts the circadian clock, acting through specialized photoreceptors in the eye that connect directly to the brain’s master clock.
  • Blue light (~460 nm) is the most effective wavelength for shifting the clock; intensity and duration also matter.
  • The phase response curve defines advance and delay zones relative to the core body temperature minimum.
  • Light before the temperature minimum delays the clock; light after it advances the clock. The largest effects occur within 3–6 hours of the minimum.
  • Eastward travel requires advances (morning light); westward travel requires delays (evening light).
  • Incorrectly timed light opposes adaptation and can trigger wrong-way shifting, substantially prolonging jet lag.
  • Intermittent bright light (~5,000 lux, 30 min on/30 min off) is nearly as effective as continuous exposure.

References

  1. Berson, D. M., Dunn, F. A., & Takao, M. (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science, 295(5557), 1070–1073.
  2. Brainard, G. C., Hanifin, J. P., Greeson, J. M., Byrne, B., Glickman, G., Gerner, E., & Rollag, M. D. (2001). Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. Journal of Neuroscience, 21(16), 6405–6412.
  3. Czeisler, C. A., Kronauer, R. E., Allan, J. S., Duffy, J. F., Jewett, M. E., Brown, E. N., & Ronda, J. M. (1989). Bright light induction of strong (Type 0) resetting of the human circadian pacemaker. Science, 244(4910), 1328–1333.
  4. Eastman, C. I., Gazda, C. J., Burgess, H. J., Crowley, S. J., & Fogg, L. F. (2005). Advancing circadian rhythms before eastward flight: a strategy to prevent or reduce jet lag. Sleep, 28(1), 33–44.
  5. Hattar, S., Liao, H. W., Takao, M., Berson, D. M., & Yau, K. W. (2002). Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science, 295(5557), 1065–1070.
  6. Khalsa, S. B. S., Jewett, M. E., Cajochen, C., & Czeisler, C. A. (2003). A phase response curve to single bright light pulses in human subjects. Journal of Physiology, 549(3), 945–952.
  7. Roach, G. D., & Sargent, C. (2019). Interventions to minimize jet lag after westward and eastward flight. Frontiers in Physiology, 10, 927.
  8. Thapan, K., Arendt, J., & Skene, D. J. (2001). An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. Journal of Physiology, 535(1), 261–267.
  9. Zeitzer, J. M., Dijk, D.-J., Kronauer, R. E., Brown, E. N., & Czeisler, C. A. (2000). Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. Journal of Physiology, 526(3), 695–702.