The influence of bright light during the daytime upon circadian rhythm of core temperature and its implications for nocturnal sleep.

The purpose of the present study was to investigate how bright light during the daytime could influence circadian rhythms of core temperature and nocturnal sleep. Seven females (age 20 +/- 2 years) served as participants. The participants lived in the experimental unit for 4 days and were exposed to either 6000 lx (bright) or 200 lx (dim) light during the daytime. Rectal temperature (T[re]) was measured during the experimental period. Subjective alertness was measured by the Kansei-gakuin Sleeping Scale five times a day. The minimum T[re] was significantly lower after bright exposure (P < 0.05). The T[re] fell rapidly after bright exposure before they retired (P < 0.05) and increased more rapidly during bright light after they woke up (P < 0.05). The morning wakefulness under bright exposure was more active than under dim exposure (P < 0.05). The melatonin secretion at wake up during bright exposure was significantly lower than during dim exposure (P < 0.05). Exposure to bright light during daytime lowered the nocturnal level of T[re], its evening fall was faster and the morning rise quicker. This suggests that indoor light during daytime should be bright enough to promote healthy sleep at night. [ABSTRACT FROM AUTHOR]