A study of more than 13 million hours of data collected from light sensors worn by 89,000 people has found exposure to bright nights and dark days is associated with an increased risk of death.

Researchers investigated whether personal day and night light and lighting patterns that disrupt our circadian rhythms predicted mortality risk.

Published in the journal Proceedings of the National Academy of Sciences, the findings indicate that individuals exposed to high levels of light at night faced a 21% to 34% increased risk of death. In contrast, those exposed to high levels of daylight experienced a 17% to 34% decrease in their risk of death.

“Exposure to brighter nights and darker days can disrupt our circadian rhythms. This disruption can lead to various health issues, including diabetes, obesity, cardiovascular disease, mental health problems, and an increased risk of death,” explains Professor Sean Cain, a senior author and sleep expert from Flinders University.

“These new insights into the potential adverse impact of light have shown us just how important personal light exposure patterns are for your health.”

Associate Professor Andrew Phillips, co-senior author, states that nighttime light exposure disrupts circadian rhythms by shifting their timing (phase-shift) and weakening the signal (amplitude suppression) of the central circadian ‘pacemaker,’ which regulates circadian rhythms throughout the body.

“Disruption to the body’s circadian rhythms is linked to the development of metabolic syndrome, diabetes, and obesity and is also strongly implicated in the development of cardiometabolic diseases, including myocardial infarction, stroke and hypertension,” says Associate Professor Phillips.

“The observed relationships of night light exposure with mortality risk may be explained by night light disrupting circadian rhythms, leading to adverse cardiometabolic outcomes.

“Our findings clearly show that avoiding night light and seeking daylight may promote optimal health and longevity, and this recommendation is easy, accessible and cost-effective,” adds Associate Professor Phillips.

The study authors from FHMRI Sleep Health investigated the relationship between personal light exposure and the risk of all-cause and cardiometabolic mortality in 89,000 participants from the UK Biobank, aged between 40 and 69. Metrics were recorded using wrist-worn sensors, and the National Health Service collected the participants’ mortality data over an approximate follow-up period of eight years.

Sleep duration, sleep efficiency, and midsleep were estimated from motion data. At the same time, cardiometabolic mortality was defined as any cause of death corresponding to diseases of the circulatory system or endocrine and metabolic diseases.

The research also showed a disrupted circadian rhythm predicted higher mortality risk, which the authors were able to determine using computer modelling. Findings accounted for age, sex, ethnicity, photoperiod, and sociodemographic and lifestyle factors.

Lead author Dr Daniel Windred says that the findings demonstrate the importance of maintaining a dark environment during the late night and early morning hours, when the central circadian ‘pacemaker’ is most sensitive to light, and seeking bright light during the day to enhance circadian rhythms.

“Protection of lighting environments may be significant in those at risk for circadian disruption and mortality, such as in intensive care or aged-care settings,” says Dr Windred.

“Across the general population, avoiding night light and seeking daylight may lead to a reduction in disease burden, especially cardiometabolic diseases, and may increase longevity.”