Health / Research
Light at Night May Matter to Your Heart
A large study using wrist-worn light sensors found people with the brightest nights had higher rates of heart disease. It does not prove light causes it, but it is a good reason to take darkness seriously.
A large study using wrist-worn light sensors found people with the brightest nights had higher rates of heart disease. It does not prove light causes it, but it is a good reason to take darkness seriously.
The study
Published in JAMA Network Open in October 2025, the study analysed nearly 89,000 UK Biobank participants who wore a light sensor on the wrist for a week. Researchers then followed their health records for up to about nine and a half years.
Compared with people who had the darkest nights, those in the brightest 10 per cent had higher rates of coronary artery disease, heart attack, heart failure, atrial fibrillation and stroke. The associations held after the researchers adjusted for a range of factors, including sleep and other health measures.
“We measure steps, sleep and heart rate. Few of us think about how dark our nights are.”
Why light might matter
Light is the main signal that sets our internal body clock. The study authors note that light at night causes circadian disruption, which is already known as a risk factor for poor cardiovascular outcomes. Night shift work, for example, has long been studied for this reason.
The new contribution is measuring real light exposure in everyday life rather than relying on questionnaires or satellite images of outdoor light.
What it does not prove
This is an observational study. People with brighter nights may differ in many ways: worse sleep, shift work, stress, health conditions, or living situations. The researchers adjusted for many of these, but no observational study can rule out every explanation.
Light was measured for only one week, and the participants were mostly middle-aged and older UK adults. So the right conclusion is not that a bedside lamp damages your heart. It is that night-time light is a plausible, under-appreciated factor worth more research.
Easy ways to darken your nights
The good news is that this is one of the cheapest health experiments available. Dim lights in the evening and use warm, low lamps instead of bright overhead light. Keep the bedroom dark with blackout curtains or an eye mask. Cover or remove glowing standby lights. Keep phones out of bed.
If you need light at night, for example for safety, keep it low and close to the floor. And get bright daylight in the morning, which helps anchor the body clock in the right direction.
Reviewed for accuracy by WT Research Desk