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Abstract

Light influences human physiology and behaviour by regulating circadian rhythms, melatonin secretion, and alertness. Previous research has reported sex differences in melatonin secretion and circadian rhythms, possibly related to women's greater sensitivity to bright light. Other studies have suggested reduced photosensitivity and earlier circadian phases in summer than in winter in mid-latitude regions. This study explores the effects of sex, seasonality, and their combination on melatonin suppression and subjective sleepiness in response to moderate light exposure, considering prior light history and menstrual phases in females. We conducted a controlled, within-subject experiment with 48 healthy adults (18-35 years, 50% female) across different seasons. The study design included two 9-h laboratory sessions, with at least 5-day washout in between. Participants were exposed to dim and moderate light through a screen for 2 hours after their habitual bedtime. Female participants exhibited greater melatonin suppression (+4.69 %) but a lower alerting response (-6.00%) to moderate light compared to males. Both sexes demonstrated increased sensitivity to the NIF effects of light in winter, with stronger melatonin suppression (+18.05 %) and increased alertness (+7.60 %) compared to summer. While prior light history did not significantly impact melatonin suppression or alertness, it was associated with an earlier dim light melatonin onset (DLMO). Females during their luteal phase had earlier DLMO than those in their follicular phase. Our findings indicate an interaction between sex, seasonality, and light exposure in modulating melatonin suppression, emphasising the need for personalised light exposure recommendations according to individual biological and environmental contexts.