Sleeping with lights on may be bad for your heart. In 73,286 UK adults who wore a light sensor, the brightest nights went with 29% more heart failure and 33% more strokes (Study). An earlier study of about 89,000 people found the same pattern, but both show a link rather than proof (Study).
If the link holds up, a lamp left on is one of the cheapest heart risks you could remove.
What Counts as a Bright Night?
Light is measured in lux, the amount of light that lands on a surface. A full moon gives less than 1 lux, and an ordinary lit room gives about 100. Streetlight through thin curtains, a hallway light under the door or a TV left on all land somewhere in between.
The newest heart study set its cutoff at just 3 lux. The authors chose that level because it may start to lower melatonin, the hormone your body makes in the dark (Study). That is very dim, but the body responds to very little light. In 55 young adults, about 25 lux was enough to cut evening melatonin in half, on average (Study). Some people were more than 50 times as sensitive as others.
Older studies mostly asked people whether they slept with a light on. The newer ones measured it, using the UK Biobank, a large UK health study that has run for many years. Volunteers wore a wrist device with a light sensor, day and night, for seven days. Across everyone, that added up to about 13 million hours of light readings (Study).
In the 2026 study, “night” meant each person’s five least active hours, whatever the clock said. Bright nights were often short. The brightest group spent about 34 minutes a night above 3 lux, on average. About 3 in 10 volunteers had no time above it at all (Study).
In the 2025 study, a typical night in the darkest half of volunteers measured about 0.6 lux. In the brightest tenth it was about 105 lux, close to a lit room (Study). All of these numbers describe light during sleep hours.
Sleeping With Lights On: Bad for the Heart?
In 2025, Windred and colleagues followed 88,905 sensor wearers for about 8 years on average. They compared people in the brightest tenth of nights with people in the darkest half. The bright group later had more heart attacks, heart failure and strokes. They also had more narrowed heart arteries and more atrial fibrillation, an irregular heartbeat (Study).
Heart failure struck 2.1% of the dark-night group and 2.9% of the bright-night group. Heart attacks struck 1.8% against 2.5% (Study).
The bright group also smoked about twice as often. So the team took smoking into account, along with income, education, alcohol, diet, exercise and city living. The gaps shrank but did not go away. Bright nights still went with about 45% higher risk of heart failure and 42% higher risk of heart attacks. Atrial fibrillation and stroke were each about 28% more likely (Study). For every outcome, risk tended to rise as nights got brighter.
Next, the team took sleep into account. Short sleep explained some of the link, but not all of it (Study). Stroke was the exception. Once short sleep or cholesterol was factored in, the stroke link became small enough that chance could explain it.
The links were stronger in women for heart failure and narrowed arteries, though not for heart attacks or stroke (Study). Women usually get heart failure less often than men. Yet among people with bright nights, women’s heart failure risk was about the same as men’s.
A year earlier, the same team looked at deaths in the same 88,905 people. Over about 8 years, the brightest tenth of nights was linked to a higher risk of dying from any cause. That risk was 21% to 34% above the darkest half (Study). The range depends on how many other factors the team took into account. The low end is the figure that allows for the most factors. Deaths from heart and metabolic disease were 33% to 46% more likely. Light around 2:30 to 3:00 am carried the highest heart and metabolic risk (Study).
Daytime light showed the opposite pattern. People in the brightest tenth for daytime light were 17% to 34% less likely to die from any cause than the darker half (Study). Part of that benefit held after allowing for exercise, which the same wrist device measured. So timing seems to matter: light by day went with lower risk, and light at night with higher risk.
Thicker, Weaker Heart Muscle on MRI
Studies like these count who got sick. They cannot show what was changing inside the heart beforehand. A 2026 study in the European Heart Journal looked at the heart itself to find out (Study).
Dai and colleagues studied 11,071 sensor wearers with no heart disease, whose average age was 61. About three years after the sensor week, each had a heart MRI, a detailed scan of the heart’s size and pumping.
People with the brightest nights had a heavier main pumping chamber than people with dark nights. Its muscle mass, allowing for height, was about 2.4% higher. Its wall was 1.5% thicker (Study). Doctors call this pattern concentric hypertrophy, meaning a heart wall that has grown thicker and heavier.
The muscle also squeezed less. One measure of how much the wall tightens with each beat was about 1.9% lower. Another measure, called strain, tracks how far the muscle shortens and stretches in three directions. It was about 3% weaker in all three (Study).
The more minutes people spent above 3 lux at night, the bigger these changes were (Study). For any one person, the changes are small and well short of a diagnosis. The authors compare their size to earlier UK Biobank scan results for air pollution. The two were measured in different ways, though.
The team then followed up to 73,286 sensor wearers for 8 to 10 years. They compared the brightest nights with nights that never went above 3 lux. People with bright nights had 29% more heart failure, 24% more heart attacks and 33% more strokes (Study). Deaths from heart disease were 26% higher, and atrial fibrillation was 15% higher.
Shorter sleep may explain part of the link. For most of the heart changes on MRI, it seemed to account for a quarter to a half (Study). The rest stayed unexplained. The authors tested this only for the scans, not for heart failure or strokes. They also warn the share could be wrong if other hidden factors connect sleep and the heart.
One Bright Night in the Lab
A small 2022 trial tested whether light during sleep can change the body overnight (Trial).
Mason and colleagues brought 20 healthy adults into a sleep lab for two nights. Their average age was about 27. Everyone slept the first night in dim light, under 3 lux. A coin flip decided who would spend the second night under ceiling lights of about 100 lux, roughly a lit room. The other 10 stayed in dim light.
Closed eyelids are thought to let through only about 5 to 9% of light (Trial). Even so, heart rate in the lit group stayed higher all night. The tiny shifts in timing between their heartbeats also tilted toward the body’s “fight or flight” mode (Trial). Daytime heart rate and blood pressure did not differ between the groups.
The next morning, the lit group showed more insulin resistance, meaning their bodies needed more insulin to handle sugar. A standard measure of it rose about 15% after the lit night. In the dim group it fell about 4% (Trial). Blood sugar levels were no different in the two groups. The people whose heart-rate pattern shifted most overnight tended to have the biggest early rise in insulin.
This was a small study: one night in 20 young, healthy people. It measured short-term signs in the body, not heart disease. The long-term effects are unknown (Trial). It shows what extra light does in a lab, but it cannot tell us whether removing light at home helps.
Other studies point the same way. Among 84,790 UK Biobank sensor wearers, the brightest nights went with about 53% higher risk of type 2 diabetes over about 8 years (Study). The main analysis did not take body weight or sleep length into account. An extra check that did still found a link.
In Japan, the HEIJO-KYO study measured night light with a meter placed in the bedroom instead of on the wrist (Study). Among 989 older adults, those with the brightest bedrooms had neck artery walls that thickened faster over about three years (Study). That thickening is an early sign of atherosclerosis, the buildup that narrows arteries. The link held after the team allowed for high blood pressure, diabetes, smoking, weight, age and income.
Why These Studies Cannot Prove Cause
Apart from the trials, every study here is observational. Researchers watched what happened, but nobody was told to sleep in a bright or a dark bedroom. The authors say plainly that this kind of study cannot show cause (Study).
Light was measured for one week, and then people were followed for about 8 years. Habits change over that time. The 2026 authors argue that this kind of error tends to hide a real link rather than create a false one (Study).
The cause may also run backwards. People who are ill, in pain, anxious or sleepless may keep a light on or get up more at night. So early, silent heart trouble could lead to bright nights, rather than the reverse. The 2026 team tried to guard against this. They left out anyone who had heart disease at the start. One extra check also dropped scans done within two years of the sensor week. And in 951 people scanned before they wore the sensor, the scans did not predict later night light (Study).
Other hidden factors could still be at work. The 2026 team allowed for a long list, including income, smoking, diet, exercise, city living, air pollution and daytime light (Study). But the sensors could not tell a phone at 2 am from a streetlight, so nobody measured screen habits (Study). Shift workers were left out of the 2026 study. Nearly all volunteers were White, and they were healthier and better off than average.
Two senior authors of the 2025 Windred heart study report ties to the lighting industry (Study). Dr Cain reports research funding from Versalux and Delos, plus consulting for Dyson, Colorbeam and Beacon Lighting. Dr Phillips reports grants from Beacon and Versalux. The two co-founded Circadian Health Innovations, a lighting company focused on the body clock. They state that these ties fall outside the study. Public grants from Australia and the US paid for the study itself.
The 2026 heart MRI study came from a separate team without these authors. It found the same pattern in the same UK Biobank data. That helps, though both studies drew on the same pool of volunteers.
Finally, as far as we could find, no trial has tested whether a darker bedroom prevents heart disease. The closest trials at home tested eye masks. One enrolled 240 pregnant women who slept 7 hours or less. A week of an eye mask plus earplugs did not lower their blood sugar or lengthen their tracked sleep (Trial). That trial did not measure the heart.
Frequently Asked Questions
Is Sleeping With Lights On or Off Better?
Off, as far as the evidence goes. In the biggest sensor studies, people with dark nights had less heart disease than people with bright nights (Study). They also died at lower rates (Study). In the lab, one lit night raised heart rate and next-morning insulin resistance (Trial). If you need some light for safety, keep it dim and low.
Does Sleeping With Lights On Affect Your Sleep?
It may change the kind of sleep you get more than how long it lasts. In the lab trial, a lit night did not shorten total sleep (Trial). But the dim group gained a bigger share of deep sleep and dream (REM) sleep than the lit group did. In the UK Biobank, bright nights went with shorter sleep. That explained a quarter to a half of the link for most heart changes on MRI (Study).
Does Sleeping With Lights On Cause Weight Gain?
It is linked to weight gain, but nobody has shown that it causes it. One study followed 43,722 US women for about six years. Those who slept with a TV or light on had a 17% higher risk of gaining 5 kg or more (Study). Among women who were not obese at the start, the risk of becoming obese was 33% higher. The light was self-reported only once, though. A one-time check of 552 older adults in Chicago found a similar link. Any light during their darkest five hours was tied to more obesity and diabetes (Study).
Is Sleeping With Lights On Bad for Kids?
No study here looked at children’s hearts, so that part is unknown. In 36 preschoolers, just 5 to 40 lux for an hour before bed cut melatonin by about 78% (Study). In a small study, bright evening light cut melatonin about twice as much in 9-year-olds as in adults (Study). These tests lasted an hour rather than a whole night. Even so, if your child needs a night light, the dimmest one that works is a sensible pick.
Key Takeaways
- Bright nights tracked with heart disease. Among 88,905 sensor wearers, people with the brightest tenth of nights had more heart disease than those with the darkest half. That included heart failure, heart attacks, atrial fibrillation and stroke (Study).
- The heart itself looked different. On MRI, bright nights went with a slightly thicker, weaker heart. Over 8 to 10 years, people with bright nights had 29% more heart failure and 33% more strokes (Study).
- One lit night changed the body. In a trial of 20 healthy adults, one night under 100 lux kept heart rate higher all night. Next-morning insulin resistance rose about 15% (Trial).
- Bright days went the other way. People with the brightest days had a 17% to 34% lower risk of dying from any cause (Study).
- The limits are real. These studies only observed people, using one week of light data. Two senior authors of the 2025 heart study have ties to the lighting industry (Study).
- The fixes are nearly free. Dark curtains, overhead lights off late and the phone out of the bedroom cost little. But the evidence comes from adding light in a lab, not from making real bedrooms darker (Trial).
Give Your Heart a Dark Night
A darker bedroom costs little, so start with the window. Blackout curtains or blinds block streetlight and early dawn. An eye mask is the cheaper option. In trials at home, masks did not make people sleep longer (Trial). One trial found small gains in memory and alertness (Trial).
Next, change the last hour of your evening. Switch off the overhead lights and use a small lamp. Move your phone out of the bedroom. If it has to stay, put it face down with notifications off, so the screen stays dark at 3 am. Our look at blue light glasses covers the questions late-night screens raise.
If you need a night light for safety, pick a dim, warm one and put it low, near the floor.
Then work on the daytime half, since bright days went with lower risk. Our guide to morning light explains how to get more of it.
For now this is a consistent link rather than proof. Until someone runs the trial, a dark bedroom is a small, sensible bet.
This article is for educational purposes and is not medical advice. Talk to a qualified clinician before changing your health regimen.
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