Sleeping with even a small amount of light in the room could raise the long-term risk of heart failure, according to a new study published in the European Heart Journal. Researchers found that people exposed to more light at night showed early signs of changes in the heart's structure and how it pumps blood.
The study analysed data from more than 11,000 participants in the UK Biobank project, a large-scale British health research database. Participants wore wrist devices that measured their exposure to light, and around three years later they underwent an MRI scan of the heart.
What the researchers found
Those who were exposed to more light overnight showed signs of heart remodelling, including a slightly thickened heart muscle and small changes in how the heart contracts. The researchers said such changes could eventually contribute to heart failure or atrial fibrillation, a type of irregular heartbeat also known as AFib.
Heart failure occurs when the heart is unable to pump blood around the body as effectively as it should, and it is a leading cause of hospital admissions among older adults worldwide. Atrial fibrillation is one of the most common heart rhythm disorders and raises the risk of stroke.
The 3-lux threshold
The researchers focused on exposure to light above 3 lux at night, a relatively low level of brightness, similar to the glow of a nightlight or light seeping in from outside. This threshold was used to split participants into two groups: those who slept in relative darkness and those who were exposed to measurable light.
The UK Biobank is one of the world's largest biomedical databases, tracking the health of half a million volunteers across the United Kingdom over many years, which allows scientists to study how everyday habits affect long-term health outcomes.
Measurable differences in the heart
The comparison between the two groups revealed several differences. Among people exposed to more light at night, the mass of the heart's left ventricle was 2.4% greater, the average thickness of the heart wall was 1.5% greater, and the heart muscle's contraction fraction, a measure of pumping efficiency, was 1.9% lower.
The researchers also recorded abnormalities in myocardial strain, a measure of how the heart muscle deforms as it beats, along with minor changes linked to the right ventricle and the left atrium.
Small changes, bigger picture
The researchers stressed that these differences were small and did not, on their own, indicate the presence of cardiovascular disease. However, they said the findings could help explain how exposure to light at night, over a long period, may contribute to the development of heart problems.
The study adds to a growing body of research examining how sleep habits affect overall health. Separate research has looked at how the type of sleep a person gets can affect their mood after waking, while a study by Chinese researchers identified an optimal sleep duration linked to a reduced risk of insulin resistance.
