Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people

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 nighttime 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 light exposure with covariate-adjusted risk for psychiatric disorders and self-harm. Greater night 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 light, greater day 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.