Nature Mental Health volume 1, pages 853–862 (2023 )Cite this article
Circadian rhythm disturbance is a common feature of many psychiatric disorders. Light is the primary input to the circadian clock, with daytime light strengthening rhythms and night-time light disrupting them. Therefore, habitual light exposure may represent an environmental risk factor for susceptibility to psychiatric disorders. We performed the largest to date cross-sectional analysis of light, sleep, physical activity, and mental health (n = 86,772 adults; aged 62.4 ± 7.4 years; 57% women). We examined the independent association of day and night-time light exposure with covariate-adjusted risk for psychiatric disorders and self-harm. Greater night-time light exposure was associated with increased risk for major depressive disorder, generalized anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm behavior. Independent of night-time light exposure, greater daytime light exposure was associated with reduced risk for major depressive disorder, PTSD, psychosis, and self-harm behavior. These findings were robust to adjustment for sociodemographics, photoperiod, physical activity, sleep quality, and cardiometabolic health. Avoiding light at night and seeking light during the day may be a simple and effective, non-pharmacological means of broadly improving mental health.
Healthy circadian rhythms are essential for mental health and wellbeing1. Many psychiatric disorders are characterized by disrupted circadian rhythms and sleep2,3. In humans, a central circadian (~24-hour) clock in the suprachiasmatic nuclei (SCN) of the hypothalamus regulates the timing of basic cellular functions4, physiology, cognition, and behavior5,6. Rhythms within the SCN are regulated by daily light-exposure patterns. This biological system evolved under predictable conditions of bright light during the day and darkness at night to ensure stable, robust rhythms7,8,9. Humans in modern, industrialized societies challenge this biology, spending ~90% of the day indoors under electric lighting10, which is dim during the day and bright at night compared with natural light/dark cycles11. Deviations from our natural light/dark cycle lead to disrupted circadian rhythms and therefore could contribute to adverse psychiatric outcomes.