Daytime bright light repeatedly boosts nighttime melatonin and keeps body clock steady in young people with fixed sleep times

Under dim light conditions (approximately 10 lx), the onset and peak of nocturnal melatonin rise were significantly delayed.

• The width of nocturnal melatonin rise was significantly shortened under dim light exposure.
• Bright light conditions (approximately 5,000 lx) did not change the phases of nocturnal melatonin rise but increased the peak level.
• Core body temperature progressively increased during dim light exposure, reaching a maximum at the transition to bright light.
• Subjective sleepiness decreased during dim light exposure and was lowest on the first day of bright light.
• Repeated daytime bright light exposure may help regulate circadian rhythms and enhance nocturnal melatonin levels.

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