Mental disorders have become the greatest burden on human health due to their high rate of disability (Friedrich, 2017; GBD 2019 Diseases and Injuries Collaborators, 2020). A possible reason for this situation is that due to the heterogeneity and complexity of the pathogenesis of mental disorders, there are still no unified consensus on the pathogenic factors and risk factors of many mental disorders. With the depth and refinement of epidemiological research, seasonal patterns have been reported in many diseases, such as autoimmune diseases, gastrointestinal diseases and cardiovascular diseases (Fares, 2013; Liao et al., 2018; Watad et al., 2017). In addition, studies have also shown that the seasonal change of environmental factors play an important role in the onset and development of mental disorders. For example, the mechanisms of maintaining body temperature have shown that mental health can be affected by environmental temperature. The potential mechanism underlying this phenomenon involves the role of neurotransmitters and biogenic amines in temperature-mediated mood regulation (Iversen, 1982). In fact, a follow-up study has confirmed the effect of seasonal changes in monoamine neurotransmitters (Matheson et al., 2015). Meyer et al. has reported seasonal courses in different cognitive processes (Meyer et al., 2016). These evidences indicate that the occurrence and development of mental disorders may show different susceptibilities to seasonal changes.

Previous studies have explored the relationship between mental disorders and seasonality. However, their conclusions are inconsistent and even contradictory owing to limitations such as small sample size, short time span of data acquisition, and information bias in the follow-up process. For example, a study suggested that depressive episodes peaked in autumn, but mania episodes were not seasonal (Silverstone et al., 1995). In contrast, in a systematic review of 51 studies, Geoffroy et al. concluded that manic episodes peaked in spring and summer followed by autumn, depressive episodes peaked in early winter and were less frequent in summer, and mixed episodes peaked in early spring or summer (Geoffroy et al., 2014). Shiloh et al. found that the admission rate of patients with schizophrenia was significantly higher in summer (Shiloh et al., 2005). Conversely, Davies et al. concluded that schizophrenia is more likely to occur during winter (Davies et al., 2000). However, Amr et al. found no significant seasonal variation in hospitalized patients diagnosed with schizophrenia (Amr and Volpe, 2012). Thus, the existing studies have failed to deliver a uniform conclusion regarding the effect of seasonal variation in mental disorders.

For a better prevention of the occurrence and development of mental disorders, a comprehensive understanding of the relevant risk factors is necessary. In addition, further research on the effect of seasonality on mental disorders will help in better utilization of medical resources and providing the necessary medical support promptly. Furthermore, it may also play a guiding role in the formulation of specific treatment strategies such as phototherapy. We aimed to answer two questions through this study. First, we examined whether seasonality affects the admission rate for mental disorders. Second, if there were differences in the length of hospitalization of psychiatric patients admitted in different seasons. To the best of our knowledge, no research has been conducted on this subject in China.