Daylight during winters and symptoms of depression and sleep problems: A within-individual analysis

Background: With climate change Northern areas of the globe are expected to have less daylight during winters due to less snow and more cloudiness. While wintertime has been linked to mental health problems, the role of wintertime daylight has been scarcely studied. We examined longitudinal associations for wintertime objective exposure to global radiation and self-reported daylight exposure with symptoms of depression and sleep problems. Methods: Our analytical sample included 15,619 respondents from three Swedish Longitudinal Occupational Surveys of Health (2012, 2014 and 2016). Objective exposure was global radiation (MJ/m(2), November-January and November-February). Subjective exposure was based on self-reported time spent outdoors in daylight (<1 h vs. >= 1 h, November-January). Symptoms of depression were evaluated using a six-item subscale of the (Hopkins) Symptom Checklist. Fixed-effects method with conditional logistic regression controlled for time-invariant participant characteristics by design and time-varying covariates were added into models. Results: One unit increase in the four-month averaged global radiation was associated with lower odds of depressive symptoms (OR 0.69, 95 % CI 0.52-0.91). These findings were confirmed using four-month cumulative exposure (OR 0.91, 95 % CI 0.85-0.98). Individuals reporting >= 1 h exposure to daylight during winter months were less likely to report depressive symptoms (OR 0.72, 95 % CI 0.60-0.82) compared to time when their exposure was < 1 h. Higher three-month exposure to global radiation suggested a protective association for sleep problems. Conclusion: These findings suggest that higher exposure to daylight during winters may contribute to lower likelihood of depression symptoms.