Sunshine duration and episodes of mental disorders and the inverse effect modification by ambient temperature

Although sunshine duration (SSD) has been associated with episodes of mental disorders, the existing evidence remains conflicting, and previous studies have overlooked the role of ambient temperature-a factor intertwined with sunlight exposure-in this association. The present study aimed to investigate the associations of SSD over short periods with risks of depressive, anxiety and schizophrenia episodes, and estimate the possible effect modification by ambient temperature.

Based on hospital admission data from the urban basic medical health insurance systems in China across 268 Chinese cities from January 1, 2013, to December 31, 2017, we conducted a nationwide time-stratified case-crossover study. The primary outcomes were episodes of depressive disorder, anxiety disorder, and schizophrenia. The individuals hospitalized for episodes of depressive disorder, anxiety disorder, and schizophrenia were included based on ICD-10 diagnosis codes. In addition to daily SSD, SSD increases between neighboring days (SDIN) over consecutive days were calculated to capture multi-day relative increases in SSD. Individuals served as their own controls, and the estimations were obtained using the conditional logistic regression models.

A total of 330,610, 221,332, and 817,296 hospital admissions for depressive disorder, anxiety disorder, and schizophrenia were included, respectively. After considering the effect modification by temperature, significant negative associations between SSD and hospital admissions for depressive disorder and schizophrenia were found mainly in the cool season: per 1-h increase in 13-day moving average SSD was associated with decreases of 3.33% (95% CI: -5.85%, -0.73%) and 2.60% (95% CI: -4.21%, -0.96%) in hospital admissions for depressive disorder and schizophrenia, respectively. Ambient temperature showed inverse modification on the SSD-admission associations. When stratified by temperature, significant negative associations between SSD and hospital admissions for depressive disorder and schizophrenia were observed in low temperature stratum (< median), whereas no significant association was found in the high temperature stratum (≥ the median). Additionally, increases in 13-day moving average SDIN were associated with significant additional decreases in admissions for depressive disorder and schizophrenia.

Shot-term exposure to increasing SSD was associated with decreases in episode risks of depressive disorder and schizophrenia, and such associations were more apparent at low temperatures. Increasing the duration of sunlight exposure and reducing exposure to increasing temperature, especially in the cool season, may help reduce the burdens of depressive disorder and schizophrenia.