Human responses to bright light of different durations

Apr 25, 2012The Journal of physiology

How people respond to bright light for different lengths of time

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Abstract

Exposure to bright light for 0.2 hours can induce a 1.07±0.36 hour phase delay in the circadian rhythm.

Bright light exposure affects the timing of melatonin production in humans.
The study involved 39 young healthy participants undergoing varied durations of bright light exposure.
Phase shifts in circadian rhythm were measured by comparing melatonin onset times before and after light exposure.
Shorter light exposure durations were more effective at delaying the circadian pacemaker than longer durations.
Melatonin suppression and subjective sleepiness also increased with longer light exposure durations.

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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 duration–response 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 (∼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.

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Human responses to bright light of different durations

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