Virginia J. Vitzthum

Seasonal variation in photoperiod may affect psychosocial and physical well‐being in healthy persons. We tested this hypothesis in healthy pre‐menopausal women, without a history of mood disorders, living year‐round in Reykjavik, Iceland (64.1°N). Participants reported daily self‐assessments of well‐being throughout a complete ovulatory menstrual cycle in summer and/or winter (70% participated in both seasons). Scores for mood, cognitive acuity, social support, physical health and a composite of these four indicators were each significantly higher in summer than in winter (linear mixed effects models: p < .001 for each model); tiredness did not differ by season. The effect of season was not significantly changed by inclusion of body mass index and/or age as covariates. Some prior studies have been hampered by sparse time sampling, inattention to covariates and/or relying on recalled data. This is to our knowledge the first investigation to test the study hypothesis with daily real‐time data spanning complete ovulatory menstrual cycles in each of two seasons. This dense sampling has revealed modest seasonal variation in well‐being in healthy women. Daylength (sunlight exposure) is likely a major, but not necessarily sole, factor in these seasonal differences in well‐being; temperature is likely less important given Iceland's relatively moderate (for its high latitude) seasonal temperature swings.