Bright nights may increase risk of death, Alzheimer's

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.

An international team of researchers, led by Flinders University, investigated whether personal day and night light, and lighting patterns that disrupt our circadian rhythms, predicted mortality risk. The researchers examined the relationship of personal light exposure with all-cause and cardiometabolic mortality risk in 89,000 UK Biobank participants, who were between 40 and 69 years old. Metrics were recorded with wrist-worn sensors and participant mortality was captured by the National Health Service across a follow-up period of approximately eight years.

The results of the study, published in the journal Proceedings of the National Academy of Sciences, show that those who were exposed to high levels of light at night had a 21–34% increased risk of death. Those exposed to high levels of daylight had a 17–34% reduction in their risk of death.

“Exposure to brighter nights and darker days can disrupt our circadian rhythms; a disruption that is known to lead to various health issues — including diabetes, obesity, cardiovascular disease [and] mental health issues — and increase the risk of death,” said senior author Professor Sean Cain, 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.”

Co-senior author Associate Professor Andrew Phillips, also from Flinders University, explained that light exposure at night disrupts circadian rhythm — that is, the body’s natural sleep–wake pattern — by shifting the timing (phase shift) and weakening the signal (amplitude suppression) of the central circadian ‘pacemaker’ that orchestrates circadian rhythms throughout the body. This can promote inflammation and make a person less resilient and more prone to disease.

“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,” Phillips said.

“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.”

The research showed that 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.

Flinders University’s Dr Daniel Windred, who was lead author on the study, said that the findings demonstrate the importance of maintaining a dark environment across 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 our circadian rhythms.

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

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

Meanwhile, a separate study by Rush University System for Health has found that outdoor light exposure in the evening may be a significant risk factor in Alzheimer’s disease. While light pollution is associated with increased risk of some disorders and diseases, this is believed to be the first time it has been associated with Alzheimer’s.

In studying US light pollution maps, Rush researchers looked at the lower 48 states and incorporated medical data associated with Alzheimer’s disease risk factors and divided the groups by light intensity. In the five groups, they found that light intensity was correlated with Alzheimer’s disease prevalence even when some well-established disease factors were not. Their results were published in the journal Frontiers in Neuroscience.

“Our research shows that there is an association in the US between Alzheimer’s disease prevalence and exposure to light at night, particularly in those under the age of 65,” said lead investigator Dr Robin Voigt-Zuwala. “Nightly light pollution — a modifiable environmental factor — may influence risk for Alzheimer’s.”

While the cause is unknown, higher night-time light intensity was associated with a greater Alzheimer’s disease prevalence than any other risk factor examined in the study for those under the age of 65, suggesting that younger people may be more sensitive to the effects of light exposure at night.

“Certain genes can influence early-onset Alzheimer’s, and these same genes may cause increased vulnerability to the effects of night-time light exposure,” Voigt-Zuwala said. “Additionally, younger people are more likely to live in urban areas and have lifestyles that may increase exposure to light at night.”

The authors acknowledged that their research results are limited to a subset of the population and that further testing is needed to better understand the connection between evening outdoor light and Alzheimer’s disease. The researchers also did not examine light inside the home at night or how it might impact health.

“The good news is that simple changes can be made with minimal effort to reduce exposure to light at night — adding blackout curtains or sleeping with an eye mask,” Voigt-Zuwala said.

Image credit: iStock.com/Tero Vesalainen