Six healthy male subjects aged 21-35 years participated in the present
study. The subjects were exposed to dim light (150 lux) or bright lig
ht (3000 lux) at eye level, from 19.00 to 21.30 h for 5 days, Rectal t
emperature and wrist activity were monitored throughout the study peri
od. Rectal temperature nadir was delayed significantly after che brigh
t light exposure. Ease in sleep initiation and overall sleep duality,
measured by questionnaire, were aggravated significantly by the evenin
g bright light exposure. These results suggest that strong illuminatio
n at night may disturb nocturnal sleep.