By the time you are reading this, you must be aware deep inside how important sleep is for health. Sleep is not just a period of rest; it is a vital biological process that allows the body to repair itself, regulate hormones, and restore energy. For many, sleep is like a see-saw; some struggle to fall asleep, others cannot stay asleep long, while some sleep excessively. Yet, beyond duration, the environment in which we sleep plays a crucial role in overall health.
A recent study has revealed that people exposed to light while sleeping may face an increased risk of heart disease.
Considering the global burden of cardiovascular diseases (CVDs), this finding is particularly significant. CVDs remain the leading cause of death worldwide, with an estimated 19.8 million people dying from these diseases in 2022, representing roughly 32 per cent of all global deaths. Among these, 85 per cent were due to heart attack and stroke. With such alarming numbers, understanding even subtle risk factors, such as exposure to light during sleep, could offer an accessible and practical strategy to reduce cardiovascular risk.
Night-time light and heart disease: Study findings
This new prospective cohort study published in ‘JAMA Network’ explored the relationship between light exposure during sleep and the risk of cardiovascular diseases among adults. The study included nearly 89,000 participants aged 41 and above, who wore light sensors on their wrists to track night-time light exposure. This approach allowed researchers to gather objective data on participants’ night-time environment rather than relying solely on self-reports, which are often prone to recall bias.
The study, conducted over 9.5 years, analysed around 13 million hours of light exposure data. Participants were grouped based on the intensity of light they were exposed to at night and researchers then examined the incidence of coronary artery disease, myocardial infarction (heart attack), heart failure, atrial fibrillation, and stroke using UK National Health Service records.
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The results were striking. Individuals with the highest exposure to night-time light had significantly higher risks of cardiovascular events.
Compared with those who slept in complete darkness, participants in the brightest light category faced a 32 per cent higher risk of coronary artery disease, a 47 per cent higher risk of heart attack (myocardial infarction), and a 56 per cent higher risk of heart failure. They also had a 32 per cent higher risk of atrial fibrillation and a 28 per cent higher risk of stroke.
The study emphasised that these associations persisted even after accounting for established cardiovascular risk factors, including physical activity, smoking, alcohol consumption, diet, sleep duration, socioeconomic status, and genetic predisposition. “Night light exposure was a significant risk factor for developing cardiovascular diseases among adults older than 40 years,” the authors wrote, highlighting the potential public health implications of this simple, modifiable factor.
Professor Frank Scheer, a neuroscientist at Brigham and Women’s Hospital in Boston and one of the study’s authors, noted, “There is no harm in keeping the bedroom completely dark,” he explained. “In fact, it is extremely beneficial for overall health.”
Interestingly, the study also found variations based on sex and age. Larger-magnitude associations of night light with risks of heart failure and coronary artery disease were observed in females, while younger adults showed stronger associations with heart failure and atrial fibrillation.
“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,” the study concluded.
Limitations of the study
While the study is groundbreaking in its scope and methodology, the researchers acknowledged several limitations. First, the UK Biobank cohort used for this research is predominantly White (97 per cent) and overrepresents individuals with higher education and income levels, as well as healthier participants, which may limit the generalizability of the findings.
Second, night-time light exposure was measured over a single week, which may not fully capture long-term exposure patterns. “Longer-term within-individual light tracking would improve our estimation of cardiovascular-light associations,” the authors noted. However, the study found that light exposure remained fairly consistent among participants with repeated measurements, lending credibility to the findings.
Third, information about the sources of light was unavailable, meaning researchers could not adjust for behavioural factors, such as watching television or using smartphones before or during sleep. Additionally, some covariates included in the analysis, such as physical activity, may themselves influence the causal pathway between light exposure and cardiovascular outcomes.
Lastly, the observational nature of the study means that while a strong association was found, causality cannot be definitively established. The authors stressed the need for “long-term circadian-informed lighting interventions for reducing cardiovascular disease risk” to determine whether minimising night-time light exposure could actively prevent heart disease.
Why it matters
According to the World Health Organisation, an estimated 17.9 million people died from CVDs in 2016, representing roughly 32 per cent of all global deaths. Of these deaths, 85 per cent were due to heart attack and stroke. Low- and middle-income countries bear a disproportionate burden, with raised blood pressure being one of the most important contributing factors. In India, for example, 63 per cent of total deaths in 2016 were attributed to non-communicable diseases, of which 27 per cent were due to cardiovascular conditions.
The relationship between artificial light exposure and health is not limited to cardiovascular outcomes. A 2020 study in West Bengal, India, found that individuals living in urban areas with higher levels of night-time radiance reported poorer sleep quality compared to their rural counterparts. The study included 263 urban and 249 rural participants and used geotagged radiance data alongside sleep quality scores measured by the Pittsburgh Sleep Quality Index (PSQI). Researchers found that high exposure levels were associated with significantly worse sleep outcomes, especially in urban populations.
Beyond sleep, artificial light at night (ALAN) has been linked to metabolic disruption, mental health challenges, and mood disorders. A 2024 narrative review of 18 studies highlighted how ALAN affects circadian physiology, potentially disrupting melatonin production and contributing to psychiatric conditions. Sleep disturbances induced by light exposure were found to be a central factor in mood alterations, underscoring the interconnected nature of sleep, environmental factors, and health outcomes.
The review concluded, “Sleep quality is a crucial determinant of wellbeing, linked not only to individual factors but also to the environment in which the person lives. For this reason, the role of light pollution as a possible moderator of the occurrence of sleep disturbances as an effect of urbanisation processes may be crucial.”
Returning to cardiovascular outcomes, the 2025 follow-up study reinforces the notion that circadian disruption caused by night-time light is a measurable risk factor for heart disease. “In this cohort study, night light exposure was a significant risk factor for developing cardiovascular diseases among adults older than 40 years,” the authors concluded, further emphasising that reducing light at night may be a simple, low-cost strategy for improving sleep and heart health.
This story is done in collaboration with First Check, which is the health journalism vertical of DataLEADS.