Phase Shifting: Advancing vs Delaying the Clock
Introduction
Crossing time zones forces the circadian system to shift its timing, either earlier (an advance) or later (a delay), to match the new local day. This process is called a phase shift. Understanding how phase shifts work, what limits their speed, and why one direction is harder than the other is essential for managing jet lag effectively.
What Is a Phase Shift?
A phase shift is a change in the timing of the circadian clock, specifically, a change in when key biological events occur relative to local clock time.
Researchers track the position of the internal clock using two key markers:
- Dim-light melatonin onset (DLMO): the time at which the body begins releasing melatonin, the hormone that signals nighttime, under dim lighting conditions. This typically occurs about 2 hours before habitual bedtime. DLMO is considered the most reliable clinical measure of where the internal clock is positioned.
- Core body temperature minimum (CBTmin): the lowest point of the daily body temperature cycle, occurring approximately 2 hours before habitual wake time. Light exposure near this temperature minimum produces the largest clock shifts; light before it produces delays, light after it produces advances.
When you travel east and need to fall asleep and wake up earlier than your body clock currently allows, you need a phase advance, shifting both melatonin onset and the temperature minimum to earlier times. When you travel west and need to stay up and wake later, you need a phase delay.
Advancing the Clock
Phase advances are required after eastward travel. Three tools can produce them:
Morning light, Bright light in the hours after the core body temperature minimum (the biological morning, which may correspond to late morning or midday at the destination if you have just arrived from the west) advances the clock. Intermittent bright light during this window, 30 minutes on, 30 minutes off, is as effective as continuous exposure and more practical (Eastman et al., 2005).
Evening melatonin, Melatonin taken as a supplement in the biological afternoon or evening, roughly 5–7 hours before sleep onset, advances the clock. The melatonin phase response curve is roughly the opposite of the light curve: it advances the clock when taken before the biological nighttime nadir and delays it afterward (Revell et al., 2006).
Gradual sleep schedule advance, Moving bedtime and wake time earlier by about 1 hour per day before or after travel keeps behavior aligned with the shifting clock and reduces sleep deprivation during adaptation (Eastman et al., 2005).
Combining all three is more effective than any single strategy. A controlled study found a median advance of 1.4 hours over 3 days using a 1 hour/day sleep advance together with intermittent morning bright light. Adding afternoon melatonin (3 mg) increased the advance rate to approximately 1 hour per day (Eastman et al., 2005; Revell et al., 2006).
Delaying the Clock
Phase delays are required after westward travel and are generally easier to achieve than advances. Two tools are effective:
Evening light, Bright light in the hours before the core body temperature minimum, the biological evening, delays the clock. Staying in well-lit environments, going outdoors in the evening, or using a light box after westward travel can accelerate delay adaptation.
Morning melatonin, Low-dose melatonin (0.5 mg) taken shortly after waking, when the biological clock is still in its advance zone, has been shown to produce delays (Burgess et al., 2010). This works through the same phase-shifting mechanism, but applied at the delay portion of the melatonin response curve.
Biological Limits on Phase Shifting
The circadian system cannot shift arbitrarily fast. Early field studies suggest the clock can delay by approximately 1.5h per day and advance by approximately 1h per day under natural, unmanaged conditions (Aschoff, 1975).
Under optimized laboratory protocols using intense light and/or melatonin, advances of 36–108 minutes per day have been achieved depending on the protocol. Practical rates achievable with real-world light management fall between these extremes: delays of approximately 2 hours per day and advances of approximately 1.5 hours per day (Eastman & Burgess, 2009).
The asymmetry between delays and advances reflects fundamental biology: the intrinsic period of the human clock is slightly longer than 24 hours (~24.2 hours), making it naturally easier to shift later (delay) than earlier (advance).
Why Gradual Shifts Are Necessary
Attempting to shift sleep too aggressively, faster than the circadian system can follow, creates a mismatch between your schedule and your biology. When sleep is advanced faster than the clock, the internal drive to sleep has not yet shifted to match the new bedtime. The result: difficulty falling asleep and poor sleep quality.
Eastman et al. (2005) demonstrated this directly. Participants who advanced their sleep schedule by 2 hours per day (twice the recommended rate) developed significant sleep difficulties by day 3. Those who advanced by 1 hour per day maintained good sleep quality throughout. The recommended maximum is approximately 1 hour per day for advances.
For delays, the mismatch is less acute because the clock’s natural drift is already in the delay direction, but shifting more than 2 hours per day still risks accumulating sleep debt.
Typical Rates of Adaptation
| Condition | Delay | Advance |
|---|---|---|
| Unmanaged (Aschoff, 1975) | ~1.5h/day | ~1h/day |
| Optimized lab protocols | ~108 min/day | 36–108 min/day |
| Practical with light management | ~2 h/day | ~1.5 h/day |
| Recommended sleep schedule shift | ≤2 h/day | ≤1 h/day |
These figures assume active management of light exposure and sleep timing. Passive adaptation, relying on uncontrolled light exposure at the destination, is substantially slower and less predictable.
Key Takeaways
- A phase shift is a change in the timing of the internal clock relative to local time.
- Advances (for eastward travel) use morning light, evening melatonin, and gradual sleep schedule advancement.
- Delays (for westward travel) use evening light and optionally morning melatonin.
- The circadian system can delay faster than it can advance, because the clock’s natural period is slightly longer than 24 hours.
- Practical adaptation rates: approximately 2 hours/day for delays, 1.5 hours/day for advances with active light management.
- Advancing sleep faster than about 1 hour/day causes sleep problems because the clock cannot keep pace with the schedule change.
- Combining morning light and evening melatonin achieves larger advances than either alone.
References
- Aschoff, J. (1975). Circadian rhythms: Influence of internal and external factors on the period measured in constant conditions. Zeitschrift für Tierpsychologie, 49(3), 225–249.
- Burgess, H. J., Revell, V. L., Molina, T. A., & Eastman, C. I. (2010). Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. Journal of Clinical Endocrinology & Metabolism, 95(7), 3325–3331.
- Eastman, C. I., & Burgess, H. J. (2009). How to travel the world without jet lag. Sleep Medicine Clinics, 4(2), 241–255.
- 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.
- 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.
- Revell, V. L., Burgess, H. J., Gazda, C. J., Smith, M. R., Fogg, L. F., & Eastman, C. I. (2006). Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light. Journal of Clinical Endocrinology & Metabolism, 91(1), 54–59.