Seasonal Effects on Hospitalizations Due to Mood and Psychotic Disorders: A Nationwide 31-Year Register Study.

To examine seasonal patterns of hospital admissions due to mood and psychotic disorders and to investigate whether the admission rates show variation according to the seasonal daylength (photoperiods). A retrospective nationwide register-based cohort of all psychiatric admissions (N=978,079) during 1987-2017 in Finland was utilized. The smoothed time-series of adjusted ratio of observed and expected (O/E) daily counts were estimated to examine seasonal variation. The mean O/E with 95% confidence intervals (CI) was used to study the admission rates by photoperiods. The calendar days were classified into the 71-day photoperiods based on the daylength (long/summer, short/winter, equal/spring, equal/fall) and the pace of change in daylength (slowly/rapidly increasing/decreasing daylength). Manic episodes peaked in summer during the long (mean O/E=1.10, 95% CI=1.06-1.13) and slowly decreasing (1.09, 1.06-1.13) photoperiods and had a nadir in winter during the slowly increasing (0.93, 0.89-0.98) photoperiod. Admissions for unipolar depressive (UPD) episodes peaked in autumn and in spring at the end of the rapidly decreasing (1.03, 1.02-1.04) and increasing (1.03, 1.01-1.04) photoperiod, and dropped in summer during the long and slowly decreasing (0.95, 0.94-0.96) photoperiods. Bipolar depressive (BPD) and mixed episodes signaled excess admissions in autumn and in spring. Admissions for schizophrenia were higher than expected from summer to early-autumn, during the long and slowly decreasing photoperiods (1.02, 1.02-1.03), and lower than expected in other seasons, especially in mid-spring during the rapidly increasing photoperiod (0.98, 0.98-0.99). The study indicates the seasonality and photoperiodicity of mental disorders, especially for manic episodes. The seasonal pattern is similar between schizophrenia and manic episodes, and between UPD, BPD, and mixed episodes.

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© 2022 Törmälehto et al.

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Tanja Svirskis has received salary from Orton Ltd and Hilla Ltd, where she has worked as a psychiatrist, and speaker and consultant honoraria or travel support from Janssen-Cilag Ltd, H. Lundbeck Ltd, Otsuka Pharmaceutical Ltd, Recordati S.p.A. and Sunovion Pharmaceuticals outside the submitted work. Mrs Soili Törmälehto and Professor Marianna Virtanen report grants from the Academy of Finland, during the conduct of the study. All other authors have no competing interests to declare that are relevant to the content of this article.