Differential effects of bright light and social cues on reentrainment of human circadian rhythms

Feb 1, 1995The American journal of physiology

Bright light and social signals affect how human body clocks adjust differently

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Abstract

Seven of nine subjects showed an orthodromic phase shift in circadian rhythm under bright light conditions.

The study assessed the reentrainment of human circadian rhythms under two conditions: with and without bright light.
A significant phase shift in circadian rhythms was observed, with a larger shift occurring with bright light pulses.
The maximum phase-advance shift occurred when bright light was applied approximately 4 hours after melatonin levels began to rise.
Ordinary room light applied at the time corresponding to bright light also produced a similar maximum phase shift but at 1 hour after melatonin onset.
Bright light and social schedules are associated with different effects on the reentrainment of circadian rhythms.

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Reentrainment of human circadian rhythm to an 8-h advanced schedule of sleep and social contacts was assessed under two different conditions: with and without bright light (4,000-6,000 lx). Subjects spent 15 days without knowing the natural day-night alternation. On the fourth day, the social schedule was phase-advanced by 8 h. In one experiment, a bright light pulse of 3-h duration was given in every subjective morning, and in the other no light pulse was applied. Plasma melatonin and rectal temperature rhythms were measured. Seven of nine subjects showed an orthodromic phase shift, the rate of which was significantly larger with bright light pulses than without them. The maximum phase-advance shift by three consecutive light pulses was observed when the first pulse was applied approximately 4 h after the onset of melatonin rise. By contrast, the maximum phase shift of a similar extent was detected at 1 h after the onset of melatonin rise, when ordinary room light (300-500 lx) at the time corresponding to bright light was regarded as a dim light pulse. It is concluded that bright light accelerates the reentrainment of human circadian rhythm, and bright light and social schedule have differential effects on the reentrainment.

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Differential effects of bright light and social cues on reentrainment of human circadian rhythms

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