Daytime illumination shapes sex differences in sleep and circadian responses to nighttime light

Abstract Light is the primary environmental cue regulating circadian rhythms and sleep–wake behavior, yet daylight intensity is rarely standardized or reported. This variability may contribute to inconsistent findings on sex differences in sleep regulation. In prior work, we observed no sex differences in sleep–wake architecture under relatively bright daylight, suggesting that daytime illumination may be a critical but underappreciated determinant of sex difference in sleep regulation. Here, we tested whether daylight intensity modulates sex differences in sleep–wake architecture and vulnerability to dim light at night (DLaN). Male and female C57BL/6 J mice were exposed to acute (one night) or chronic (two weeks) DLaN (10-lux) under daytime light intensities of 50-, 100-, or 300-lux. Sleep was assessed using electroencephalographic measures of vigilance states and slow wave activity (SWA). Dim daylight (50-lux) revealed marked sex differences in dark phase that were absent under brighter daylight (300-lux). Acute DLaN reduced early-night wakefulness in both sexes under low daytime light but had little effect under bright daylight conditions. Following chronic DLaN, males showed reduced wakefulness and dampened rhythm amplitude, whereas females exhibited larger phase shifts, rhythm attenuation, and altered SWA timing under 50- and 100-lux, and bright daytime light largely prevented these effects.