Light Exposure as a Modifiable Determinant of Mental Health

Light is a fundamental environmental signal that shapes human physiology, behaviour, and mental health. Beyond vision, light exposure regulates circadian rhythms, sleep, neuroendocrine function, arousal, and brain circuits implicated in emotional regulation. This review synthesizes recent evidence linking light exposure to mental health and argues that light should be conceptualized as a core, modifiable component of the mental health exposome – the cumulative, dynamic set of environmental influences shaping mental health across the lifespan. Recent randomized controlled trials using light therapy, large-scale epidemiological studies, and neurophysiological investigations demonstrate that habitual patterns of daytime and nighttime light exposure are associated with a broad range of mental health outcomes. Higher daytime light exposure is generally associated with better mood and lower depressive symptomatology, whereas greater exposure to light at night is linked to increased risk of depression, anxiety, and sleep disturbance. Advances in wearable light measurement and digital phenotyping now enable precise characterization of individual light environments, supporting observational studies and improving dose verification in light-based interventions. Emerging neurobiological evidence further suggests that light can influence affective brain circuits through pathways that extend beyond sleep and circadian regulation. Together, converging evidence positions light exposure as a biologically potent and highly modifiable determinant of mental health operating across multiple temporal scales, from acute alerting effects to longer-term circadian and behavioural adaptation. Conceptual challenges remain, including bidirectionality between light exposure and mental health and limitations in causal inference. Nevertheless, improved measurement technologies and personalized, just-in-time intervention strategies open new opportunities for integrating light exposure into psychiatric research, prevention, and clinical practice.