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Updated: Mar 17, 2026

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Can Morning Light Phase Advance Human Melatonin Rhythms in Less Than 24 h?

Clara López-Velasco1,2, Carolin Franziska Reichert1,2, Christian Cajochen1,2

  • 1Centre for Chronobiology, Psychiatric Hospital of the University of Basel, Basel, Switzerland.

Journal of Pineal Research
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PubMed
Summary

Morning light can advance human circadian rhythms within 24 hours. Modest phase advances (10-30 minutes) are achievable the same day with bright, blue-enriched light exposure shortly after waking.

Keywords:
DLMOcircadian rhythmlightmelanopic EDImelatoninphase advancephase response curve

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Area of Science:

  • Chronobiology
  • Sleep Science
  • Light Therapy

Background:

  • The human circadian system synchronizes to the 24-hour day primarily through light cues.
  • Morning light is known to induce phase advances in circadian rhythms, but the timing of this effect (same-day vs. next-day) is often unclear.
  • Most studies assess melatonin onset over 24 hours post-intervention, leaving a gap in understanding same-day effects.

Purpose of the Study:

  • To determine if morning light can phase-advance human melatonin rhythms in less than 24 hours.
  • To review studies utilizing same-day or single-cycle protocols for light interventions.
  • To investigate the relationship between light intensity (melanopic equivalent daylight illuminance - mEDI) and the magnitude of phase advance.

Main Methods:

  • Conducted a narrative review of studies assessing light exposure and dim-light melatonin onset (DLMO) within the same 24-hour period.
  • Estimated melanopic equivalent daylight illuminance (mEDI) for light interventions to enable cross-study comparisons.
  • Analyzed data for trends associating mEDI with the magnitude of observed phase advances.

Main Results:

  • Modest phase advances of 10-30 minutes were observed within the same circadian cycle when light exposure occurred shortly after waking.
  • Effectiveness of phase advance was enhanced by bright, blue-enriched light or longer exposure durations (>1 hour).
  • A trend (r=0.51, p=0.06) indicated a positive association between mEDI and the magnitude of the phase advance.

Conclusions:

  • Same-day phase advances in human melatonin rhythms by morning light appear possible.
  • These effects are not yet well-characterized, necessitating further targeted research.
  • Confirmation could shorten circadian protocol lengths, potentially accelerating clinical applications for circadian rhythm sleep-wake disorders.