Human responses to bright light of different durations

Key points Light is the strongest time cue for entrainment and phase resetting of the circadian clock. In humans, exposure to long-duration light (6.5 h) in the late evening/early night causes phase delays, suppresses melatonin and increases alertness. Here we studied the effects of different durations of exposure to a single high-intensity (similar to 10,000 lux) light pulse (0.2 h, 1 h, 2.5 h and 4.0 h) on phase shifting, suppression of melatonin and self-reported sleepiness in young men and women. Phase-resetting and melatonin-suppression responses were dose dependent and non-linear; shorter light exposures more efficiently phase-shift the clock, suppress melatonin and induce alertness. Abstract Light exposure in the early night induces phase delays of the circadian rhythm in melatonin in humans. Previous studies have investigated the effect of timing, intensity, wavelength, history and pattern of light stimuli on the human circadian timing system. We present results from a study of the durationresponse relationship to phase-delaying bright light. Thirty-nine young healthy participants (16 female; 22.18 +/- 3.62 years) completed a 9-day inpatient study. Following three baseline days, participants underwent an initial circadian phase assessment procedure in dim light (<3 lux), and were then randomized for exposure to a bright light pulse (similar to 10,000 lux) of 0.2 h, 1.0 h, 2.5 h or 4.0 h duration during a 4.5 h controlled-posture episode centred in a 16 h wake episode. After another 8 h sleep episode, participants completed a second circadian phase assessment. Phase shifts were calculated from the difference in the clock time of the dim light melatonin onset (DLMO) between the initial and final phase assessments. Exposure to varying durations of bright light reset the circadian pacemaker in a dose-dependent, non-linear manner. Per minute of exposure, the 0.2 h duration was over 5 times more effective at phase delaying the circadian pacemaker (1.07 +/- 0.36 h) as compared with the 4.0 h duration (2.65 +/- 0.24 h). Acute melatonin suppression and subjective sleepiness also had a dose-dependent response to light exposure duration. These results provide strong evidence for a non-linear resetting response of the human circadian pacemaker to light duration.