["American Journal of Human Biology, Volume 38, Issue 4, April 2026. ", "\nABSTRACT\n\nObjective\nThis study investigated the relationship between morning cortisol concentration and circadian timing of physical activity, light exposure and sleep in Arctic residents, and examined the role of the MTNR1B rs10830963 polymorphism on this relationship.\n\n\nMethods\nCircadian parameters were derived from 7‐day actigraphy utilizing the ActTrust 2 device with RGB light sensors. Actigraphy and cortisol were assessed in the same 27 (21 women) adult participants longitudinally during winter and summer solstices and spring equinox. Associations with morning cortisol were assessed by correlation and regression analyses, stratified by indigeneity, adjusting for photoperiod, age, and sex. A two‐way ANOVA assessed the effects of season and MTNR1B genotype on cortisol.\n\n\nResults\nLater timing of physical activity (β = 0.374, p < 0.001), light exposure (β = 0.242, p = 0.028), and bedtime (β = 0.345, p = 0.001) were significantly associated with higher morning cortisol. Stratification revealed that Nonnatives were primarily affected by the timing of light exposure (β = 0.321, p = 0.013), whereas Natives exhibited associations only with the timing of activity and sleep (β = 0.619, p = 0.002), but not light exposure. The MTNR1B rs10830963 genotype modulated cortisol, with G‐allele carriers showing lower concentrations.\n\n\nConclusion\nCortisol in Arctic residents is linked to the timing of circadian behavior, with population‐specific drivers: Nonnatives integrate both photic and behavioral cues, while the indigenous population relies on behavioral rhythms. MTNR1B polymorphism further modulates cortisol, indicating that environmental timing and genetic background regulate activity of the Hypothalamic–Pituitary–Adrenal axis in extreme photic environments.\n\n"]