Bright light can delay human temperature rhythm independent of sleep.

New evidence suggest that the phase of human circadian (approximately 24 h) rhythms can be reset by exposures to bright artificial light. This study explored the value of 5 h of bright 3,800-6,000 lx light in resetting human circadian rhythm phases among healthy volunteers. Nine subjects were exposed to 3-5 h bright light for three evenings and to control dim light for an equivalent time, in counterbalanced orders. Electroencephalogram sleep recordings, overnight rectal temperature measurements, and mood scales were obtained. Compared with control dim light, bright light delayed the body temperature rhythm approximately 3 h; however, sleep latency was delayed only 9 min. This suggests that bright light shifted the phase of the human circadian temperature rhythm independent of its effects on sleep phase. Resetting rhythms with bright light may prove useful in treatment of psychiatric disorders.