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Soili Törmälehto,1 Tanja Svirskis,2 Timo Partonen,3 Erkki Isometsä,2 Sami Pirkola,4 Marianna Virtanen,1,5 Reijo Sund6 1School of Educational Sciences and Psychology, University of Eastern Finland, Joensuu, Finland; 2Department of Psychiatry, University of Helsinki and Helsinki University Hospital, Helsinki, Finland; 3Department of Public Health and Welfare, Finnish Institute for Health and Welfare, Helsinki, Finland; 4Faculty of Social Sciences, University of Tampere and Pirkanmaa Hospital District, Tampere, Finland; 5Division of Insurance Medicine, Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden; 6Institute of Clinical Medicine, University of Eastern Finland, Kuopio, FinlandCorrespondence: Soili Törmälehto, School of Educational Sciences and Psychology C/O Institute of Clinical Medicine, University of Eastern Finland, P.O. Box 1627, Kuopio, FI-70211, Finland, Email soili.tormalehto@uef.fiPurpose: 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).Patients and Methods: 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).Results: 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).Conclusion: 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.Keywords: seasonal variation, photoperiod, hospital admission, mood disorders, bipolar disorder, schizophrenia

Aims. Although seasonality has been documented for mental disorders, it is unknown whether similar patterns can be observed in employee sickness absence from work due to a wide range of mental disorders with different severity level, and to what extent the rate of change in light exposure plays a role. To address these limitations, we used daily based sickness absence records to examine seasonal patterns in employee sickness absence due to mental disorders. Methods. We used nationwide diagnosis-specific psychiatric sickness absence claims data from 2006 to 2017 for adult individuals aged 16-67 (n = 636,543 sickness absence episodes) in Finland, a high-latitude country with a profound variation in daylength. The smoothed time-series of the ratio of observed and expected (O/E) daily counts of episodes were estimated, adjusted for variation in all-cause sickness absence rates during the year. Results. Unipolar depressive disorders peaked in October-November and dipped in July, with similar associations in all forms of depression. Also, anxiety and non-organic sleep disorders peaked in October-November. Anxiety disorders dipped in January-February and in July-August, while non-organic sleep disorders dipped in April-August. Manic episodes reached a peak from March to July and dipped in September-November and in January-February. Seasonality was not dependent on the severity of the depressive disorder. Conclusions. These results suggest a seasonal variation in sickness absence due to common mental disorders and bipolar disorder, with high peaks in depressive, anxiety and sleep disorders towards the end of the year and a peak in manic episodes starting in spring. Rapid changes in light exposure may contribute to sickness absence due to bipolar disorder. The findings can help clinicians and workplaces prepare for seasonal variations in healthcare needs. [ABSTRACT FROM AUTHOR]