Artificial moonlight partly increases fruit flies’ night activity by directly affecting behavior outside their internal clock

All clock mutants of fruit flies shifted their activity significantly into the night under artificial moonlight conditions.

• Artificial moonlight exposure resulted in nocturnal activity in fruit flies, independent of their internal circadian clock.
• Most clock-mutant flies were more active under artificial moonlight than in complete darkness.
• Residual rhythmicity was observed in specific clock mutants, indicating some retained activity despite the lack of a functional clock.
• Dim light exposure may stimulate activity in fruit flies through mechanisms not directly linked to their circadian rhythms.
• Similar effects of nocturnal light on activity have been noted in some primates, suggesting a broader biological relevance.

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