Bright days, dark nights: translating science on the non-visual effects of light into public health guidance

Light is a daily environmental exposure that can influence circadian timing, sleep, alertness, mood, and cognition, yet public health guidance still focuses mainly on optical-radiation hazards rather than healthy patterns of visible light and darkness.

This policy brief translates recent evidence and expert consensus on the non-visual effects of ocular light exposure into public health guidance. The evidence supports a practical 24-h pattern: brighter light during the morning and daytime, lower light in the evening, and darkness or near-darkness during sleep. This principle should be communicated as general guidance rather than a universal dose, because responses vary with timing, intensity, spectrum, duration, prior light exposure, age, work schedules, health status, and geography.

Public health organizations can communicate plain-language guidance, integrate light into workplaces, schools, healthcare, elder care, housing, and public buildings, align daylight messages with sun-safety advice, and support research, measurement, and melanopic labeling.

Clear, proportionate guidance on light and darkness can help people and institutions structure daily environments to support circadian alignment, sleep, alertness, mental health, and wellbeing.