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The present study investigated the effects of evening bright light (BL) compared to dim light (DL) on next-day sleep propensity as well as the acrophase of oral temperature and mood scores. A total of 12 male subjects (mean age 23.5 +/- 2.6 years) were exposed to 2 h of evening DL or BL 30 min after sunset for 5 consecutive days. The experimental period started after the 5th day of exposure. After light exposure, subjects remained awake in the sleep laboratory until 07:00 h, when they began the 7/13 ultrashort sleep-wake paradigm, which continued for 24 h until 07:00 h the next day. Oral temperature was measured hourly. The results showed that evening exposure to BL, in comparison to DL, significantly delayed the next-day sleep gate as well as the acrophase of the oral temperature curve and the acrophase of negative mood. The effects of BL on the next-day distribution of sleep stages showed an increase of Stage 2 and less REM (rapid eye movement) during the morning. The results indicate that sleep propensity is regulated by a circadian pacemaker that is responsive to evening BL exposure.
Tzischinsky et al. (Sun,) studied this question.