How exposure to light during nighttime impacted heart health of nearly 90,000 people

It’s long been known that darkness improves the length and quality of sleep. Studies have shown that even the phase of the moon can affect our sleeping patterns.

Now a new study has revealed a link between nighttime exposure to light and cardiovascular health. The research, posted last week in the journal JAMA Network Open , studied close to 90,000 people aged 40 and older to look for links between light and various cardiovascular conditions: coronary artery disease, myocardial infarction (heart attacks), heart failure, atrial fibrillation and stroke.

The long-term study followed participants in the U.K. over a period of 9.5 years, from June 2013 to November 2022. Scientists gathered some 13 million hours of light exposure data tracked by wrist-worn sensors worn for a week at a time by participants.

The participants, 88,905 in total, were 57 per cent female and 43 per cent male, with a mean age of 62. Most (97 per cent) were white. Anyone with pre-existing cardiovascular health conditions was removed from the study.

The results found that, across all the conditions being studied, the risks were greater for those who slept in brighter conditions.

For instance, those sleeping in the brightest conditions — equivalent to having an overhead light on in the room — had a 56 per cent greater risk of developing heart failure or having a heart attack compared to those who slept in the darkest places.

Bright light also led to a 32 per cent higher risk of coronary artery disease, a 32 per cent higher risk of atrial fibrillation (irregular and often rapid heartbeat), and a 30 per cent higher risk of stroke than in the dark-sleeping group.

“These associations were robust after adjusting for established cardiovascular risk factors, including physical activity, smoking, alcohol, diet, sleep duration, socioeconomic status, and polygenic risk,” the study found.

It also noted that females were more susceptible than males for risks of heart failure and coronary artery disease after sleeping under bright light. Similarly, younger individuals were found to have a higher risk of heart failure and atrial fibrillation than older participants.

“Our findings are consistent with higher prevalence of cardiovascular risk factors in people with brighter nights, observed in smaller cohorts with objective light data,” researchers wrote. They also found results consistent with higher cardiovascular risks observed in rotating shift workers, “a population that experiences frequent exposure to bright light during the biological night.”

Several drawbacks to the study were noted, despite its size and timescale. Researchers pointed out that participants tended to have higher education levels and income than the general populace, and that the study did not identify the source of the light, “meaning we could not adjust for behavioural correlates of night light exposure (e.g., light from stimulating digital content).”

The study also did not capture the causal relationship of night light with cardiovascular disease risk. “Long-term circadian-informed lighting interventions for reducing cardiovascular disease risk are needed,” they wrote.

However, they concluded: “Night light exposure was a significant risk factor for developing cardiovascular diseases among adults older than 40 years. These findings suggest that, in addition to current preventive measures, avoiding light at night may be a useful strategy for reducing risks of cardiovascular diseases.”

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