DIFFERENTIAL-EFFECTS OF BRIGHT LIGHT AND SOCIAL CUES ON REENTRAINMENTOF HUMAN CIRCADIAN-RHYTHMS

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 da ys without knowing the natural day-night alternation. On the fourth da y, 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 morn ing, and in the other no light pulse was applied. Plasma melatonin and rectal temperature rhythms were measured. Seven of nine subjects show ed an orthodromic phase shift, the rate of which was significantly lar ger with bright light pulses than without them. The maximum phase-adva nce shift by three consecutive light pulses was observed when the firs t pulse was applied similar to 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 (30 0-500 lx) at the time corresponding to bright light was regarded as a dim light pulse. It is concluded that bright light accelerates the ree ntrainment of human circadian rhythm, and bright light and social sche dule have differential effects on the reentrainment.