Sex-specific relationship between melanopsin-dependent light sensitivity and chronotype across the lifespan

Abstract Study objectives: Light, acting primarily via melanopsin-mediated signaling, plays a central role in synchronising circadian rhythms. Individuals vary markedly in the sensitivity of their circadian system to light. Whether these differences contribute to the interindividual variability in chronotype, a behavioural manifestation of internal circadian timing, is unclear. The aim of this study was to determine the relationship between melanopsin-dependent light sensitivity and chronotype across the general population. Methods: Participants (adults and children aged ≥ 8 years) were recruited in a science museum. Chronotype was determined using the µMCTQ and the post-illumination pupillary response (PIPR) to short and long-wavelength light stimuli was used as a measure of melanopsin-dependent light sensitivity. The relationship between PIPR and chronotype and their interaction with age and sex were assessed using multiple linear regression. Results: Pupil recordings and questionnaires were available from 457 participants, including 284 adults and 173 children. In adults, the relationship between melanopsin-dependent light sensitivity and chronotype depends on sex and age: in young adult men, greater light sensitivity is linked to a significantly later chronotype, whereas it is significantly associated with an earlier chronotype in older adult women. In children, no evidence was found for a relationship between light sensitivity and chronotype. Conclusions: These findings suggest that individual variation in light sensitivity interacts with sex and age-specific differences in the circadian system and light exposure behaviour to influence circadian timing. Light exposure recommendations should be personalised to take into account these sex and age-specific effects. Statement of significance While individuals differ widely in how their circadian system responds to light, to what extent these individual responses influence internal circadian timing remains unclear. By studying a large and diverse sample of children and adults, our findings reveal that the relationship between light sensitivity and chronotype, a behavioural manifestation of circadian timing, is shaped by both age and sex, offering a more comprehensive understanding of this relationship than previously recognised. Specifically, greater light sensitivity is linked to later chronotype in young men but to earlier chronotype in older women. These results reveal that the impact of light on circadian timing changes across the lifespan and will contribute to the development of personalised light exposure guidelines to promote health and well-being.