Light-induced changes of the circadian clock of humans: Increasing duration is more effective than increasing light intensity

Karuna Dewan, Susan Benloucif*, Kathryn Reid, Lisa F. Wolfe, Phyllis C. Zee

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

75 Scopus citations


Study Objectives: To evaluate the effect of increasing the intensity and/or duration of exposure on light-induced changes in the timing of the circadian clock of humans. Design: Multifactorial randomized controlled trial, between and within subject design Setting: General Clinical Research Center (GCRC) of an academic medical center Participants: 56 healthy young subjects (20-40 years of age) Interventions: Research subjects were admitted for 2 independent stays of 4 nights/3 days for treatment with bright or dim-light (randomized order) at a time known to induce phase delays in circadian timing. The intensity and duration of the bright light were determined by random assignment to one of 9 treatment conditions (duration of 1, 2, or 3 hours at 2000, 4000, or 8000 lux). Measurements and Results: Treatment-induced changes in the dim light melatonin onset (DLMO) and dim light melatonin offset (DLMOff) were measured from blood samples collected every 20-30 min throughout baseline and post-treatment nights. Comparison by multi-factor analysis of variance (ANOVA) of light-induced changes in the time of the circadian melatonin rhythm for the 9 conditions revealed that changing the duration of the light exposure from 1 to 3 h increased the magnitude of light-induced delays. In contrast, increasing from moderate (2,000 lux) to high (8,000 lux) intensity light did not alter the magnitude of phase delays of the circadian melatonin rhythm. Conclusions: Results from the present study suggest that for phototherapy of circadian rhythm sleep disorders in humans, a longer period of moderate intensity light may be more effective than a shorter exposure period of high intensity light.

Original languageEnglish (US)
Pages (from-to)593-599
Number of pages7
Issue number5
StatePublished - May 1 2011


  • Circadian rhythm sleep disorder (CRSD)
  • Phase delay
  • Phase shift
  • Phototherapy

ASJC Scopus subject areas

  • Clinical Neurology
  • Physiology (medical)


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