Circadian rhythm disturbance is a common feature of psychiatric disorders. Light is the primary input to the circadian clock, with daytime light exposure strengthening rhythms and nighttime light exposure weakening rhythms. However, the independent effects of day and night light exposure on psychiatric outcomes have not been well characterized. In this study, we performed the largest to-date cross-sectional analysis of objectively measured day and night light exposure and examined their relationship with psychiatric disorders. UK Biobank cohort participants aged 37-73 years were recruited into the UK Biobank general cohort population. In a subset of participants (n=86,772; 43% male), light and physical activity patterns were monitored for ~7 days. Using regression models adjusted for age, sex, ethnicity, photoperiod (day length), employment, and physical activity, we examined the independent associations of day and night-time light with psychiatric disorders and measures of symptom severity. We found that greater night light exposure was associated with greater odds of major depressive disorder (MDD), generalized anxiety disorder (GAD), bipolar disorder, post-traumatic stress disorder (PTSD), self-harm behavior and psychotic experiences. Conversely, greater daytime light exposure was associated with lower odds of MDD, PTSD, self-harm behavior and psychotic experiences. There was no significant association of day light exposure with GAD or bipolar disorder. Our findings demonstrate that low day light and bright night light exposure are associated with a wide range of psychiatric outcomes. Avoiding light at night and seeking light during the day may be a simple and effective, non-pharmacological means of broadly improving mental health.