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

Sleep. 2011 May 1;34(5):593-9. doi: 10.1093/sleep/34.5.593.


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.

Keywords: Circadian rhythm sleep disorder (CRSD); phase delay; phase shift; phototherapy.

Publication types

  • Randomized Controlled Trial
  • Research Support, N.I.H., Extramural

MeSH terms

  • Adult
  • Body Temperature / radiation effects
  • Circadian Rhythm / radiation effects*
  • Dose-Response Relationship, Radiation
  • Female
  • Humans
  • Light
  • Male
  • Melatonin / blood
  • Phototherapy / methods*
  • Time Factors
  • Young Adult


  • Melatonin