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

Soili Törmälehto; Tanja Svirskis; Timo Partonen; Erkki Isometsä; Sami Pirkola; Marianna Virtanen; Reijo Sund

doi:10.2147/CLEP.S372341

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

Clin Epidemiol. 2022 Oct 21:14:1177-1191. doi: 10.2147/CLEP.S372341. eCollection 2022.

Authors

Soili Törmälehto¹, Tanja Svirskis², Timo Partonen³, Erkki Isometsä², Sami Pirkola⁴, Marianna Virtanen^{1

5}, Reijo Sund⁶

Affiliations

¹ School of Educational Sciences and Psychology, University of Eastern Finland, Joensuu, Finland.
² Department of Psychiatry, University of Helsinki and Helsinki University Hospital, Helsinki, Finland.
³ Department of Public Health and Welfare, Finnish Institute for Health and Welfare, Helsinki, Finland.
⁴ Faculty of Social Sciences, University of Tampere and Pirkanmaa Hospital District, Tampere, Finland.
⁵ Division of Insurance Medicine, Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden.
⁶ Institute of Clinical Medicine, University of Eastern Finland, Kuopio, Finland.

Abstract

Purpose: 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: bipolar disorder; hospital admission; mood disorders; photoperiod; schizophrenia; seasonal variation.

Grants and funding

The research leading to these results has received funding from the Academy of Finland (329222 and 329226). The institution had no role in the design of the study or collection, analysis and interpretation of data, or in writing the manuscript, or in the decision to submit the manuscript for publication.