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Most of us don’t think of a little light as a health problem. A phone that lights up on the bedside table. A streetlight glowing around the edge of the curtain. The small red dot on the TV. We sleep through all of it, or we think we do.
A new study suggests your heart may be keeping track.
Researchers measured how much light reached more than 11,000 people while they slept. About three years later, they scanned those people’s hearts. The ones who had been sleeping with the light on, even a fairly dim one, tended to have hearts that were thicker and a little stiffer. That is an early kind of change doctors pay attention to, because it can come before heart trouble.
It is not proof that light harms the heart. The researchers are careful about that, and so will we be. But it is the first time anyone has looked this closely, and what they found is worth knowing before you turn in tonight.
What the study found
The study was published on 9 September 2026 in the European Heart Journal. It was led by Lu Qi, an epidemiologist at Tulane University in New Orleans, and it used information from the UK Biobank, a long-running health project that follows about half a million volunteers in Britain.
Between 2013 and 2015, 11,071 adults with no history of heart disease wore a small sensor on their wrist for seven days. The sensor tracked how they moved and how much light reached them, day and night. About three years later, each person had a detailed MRI scan of the heart.
The team compared people whose nights were almost completely dark with people who were exposed to more than 3 lux of light while they slept. People in the brighter group tended to have:
- A heavier main pumping chamber. The muscle of the left ventricle, the chamber that pushes blood out to the body, was about 2.4% greater in mass.
- Thicker walls. The walls of that chamber were about 1.5% thicker, leaving a little less room inside.
- A heart that flexed less. Measures of how well the heart muscle stretches and squeezes with each beat were around 2% to 3% lower.
Those are small differences, and none of these people had heart disease when they were scanned. But the pattern, a thicker, stiffer heart with less space inside, is the kind of early change doctors call remodeling. It can be one of the first steps toward heart failure.
“This is the first study of its kind, and it shows that exposure to higher levels of light at nighttime is linked to cardiac remodeling,” Qi said.
The same paper then looked at a larger group of 73,286 people and followed what happened to their hearts over the following years. Compared with people whose nights were darkest, those whose nights were brightest had a 29% higher risk of heart failure, a 33% higher risk of stroke and a 26% higher risk of dying from heart disease.
If your nights are more restless than you’d like, and not only because of the light, there is a free guide just below on why the mind won’t slow down at night, and the simple order of steps that helps it settle.
Is sleeping with the light on bad for your heart?
Here is the honest answer: this study cannot tell us for sure.
It is what scientists call an observational study. It noticed that two things tend to go together, brighter nights and changes in the heart. It did not prove that one caused the other.
That matters, because people who sleep in brighter rooms may be different in other ways too. They may live in busy cities with streetlights outside. They may work night shifts. They may have less money for thick curtains or a quiet street. Any of those things could affect the heart on its own. Studies like this try to adjust for differences like these, but they can never rule them all out.
There is one more limit worth knowing. The light was measured over a single week, and the heart scans came about three years later. One week is only a snapshot of someone’s nights.
So nobody should panic about the night-light in the hallway. But nobody should shrug it off either. The researchers’ own conclusion was that their findings “highlight dark nights and adequate sleep for cardiovascular health.”
The detail most reports skipped
Most coverage said the study measured light “at night.” It was more careful than that.
The researchers did not simply look at the hours between, say, midnight and 5am. For each person, they used the wrist sensor to find that person’s own quietest five hours, the stretch when their body was most still. That was treated as their night, whenever it happened to fall.
So the study was really asking a question about the body, not the clock. Not “was the light on at 2am?” but “was there light while this person’s body was trying to rest?” For a night-shift worker sleeping at noon, or a new parent grabbing sleep in the early evening, that is the question that counts.
It also means the light that mattered was not bright. Three lux is dim. It is not a ceiling light. It is the kind of low glow many of us sleep with without noticing: a streetlight around the edge of a curtain, a hallway light under the door, a screen that lights up with a message. If you have been waking up tired for no clear reason, it may be one of several small things worth checking, alongside the tiny wake-ups your brain has without telling you.
Darkness as a vital sign
Professor Thomas Münzel, a heart specialist at University Medical Center Mainz in Germany, put it in a way that stops you for a second. He said that “darkness deserves recognition as a vital sign, as essential to cardiovascular health as blood pressure control and clean air.”
A vital sign. The things a nurse checks first: heartbeat, breathing, blood pressure. And now darkness, on that list.
It is a strange thing to say, because we tend to think of darkness as nothing. An absence. A gap where the light should be. Modern life treats the night as spare time we haven’t found a use for yet, so we fill it with screens and streetlights and the glow of devices that never quite switch off.
The body, it turns out, may not see it that way. The dark seems to be doing something. The heart may be counting on it.
That idea is older than it sounds. In one of the oldest creation stories people still read, the dark is not a mistake to be fixed. It is given a name, Night, on purpose, in a world that by the end of the story is called very good. And each day is counted from evening to morning. The dark comes first, and the work of the day comes after it. The people who first told that story seem to have understood something we are only now measuring with wrist sensors and MRI scans: night is not wasted time. It was meant as a gift.
How to make your bedroom darker tonight
None of this needs much money. Most of it needs about two minutes.
- Turn your phone face down, or put it across the room. A screen that lights up with every message is one of the easiest sources of night light to remove.
- Cover the little lights. Standby dots on TVs, chargers and routers can be covered with a small piece of tape.
- Close the door. A hallway or bathroom light left on can reach further than you think.
- Deal with the window. Blackout curtains help if you can get them. If not, a spare blanket or a thick cloth hung over the curtain does much of the same job.
- If you need a light, keep it low and warm. For a child who is afraid of the dark, or for getting up safely at night, a dim, warm-coloured light placed low and away from the bed is a gentler choice than a bright overhead one.
- Cover your eyes. If the room itself can’t get dark, a sleep mask, or even a soft folded cloth, can do it for you.
If light is only one part of why your nights feel broken, this free Why Can’t I Sleep? check takes a couple of minutes and helps you see which parts of your evening are getting in the way. And if you wake up tired even after a full night, here is why more sleep does not always fix it.
Common questions about sleeping with the light on
Is it bad to sleep with the light on?
A study published in the European Heart Journal on 9 September 2026 found that people exposed to more than 3 lux of light while they slept, a fairly dim level, tended to have thicker, stiffer hearts about three years later than people who slept in near-darkness. The study, led by Lu Qi of Tulane University, followed 11,071 UK Biobank participants who wore wrist light sensors for seven days. It was an observational study, so it shows a link rather than proof that light causes heart changes, but its authors concluded that the findings highlight the value of dark nights for heart health.
Can light at night cause heart disease?
Research has not proven that light at night causes heart disease, but a 2026 study in the European Heart Journal found a clear link. In an analysis of 73,286 UK Biobank participants, led by Lu Qi of Tulane University, people with the brightest nights had a 29% higher risk of heart failure, a 33% higher risk of stroke and a 26% higher risk of dying from heart disease than people with the darkest nights. Because the study was observational, other factors such as city living, night-shift work or income may explain part of the link.
How dark should a bedroom be for sleep?
There is no single official number, but darker appears to be better for the heart. In a 2026 study of 11,071 people published in the European Heart Journal, heart changes were seen in people exposed to more than 3 lux of light while they slept, which is a dim level, the kind of low glow that can come from a streetlight around a curtain or a phone screen. Simple steps such as turning phones face down, covering standby lights with tape, closing the bedroom door and using curtains or a sleep mask can make a bedroom much darker at little or no cost.
What counts as light at night?
Light at night means any light that reaches a person while their body is trying to rest, not only a lamp left on. Common sources include phone and tablet screens, televisions, standby lights on electronics, hallway lights under the door and streetlights shining through curtains. In a 2026 European Heart Journal study on light and heart health, researchers measured it during each person’s own least-active five hours rather than at fixed clock times, so light during daytime sleep after a night shift would count too.
A question worth sitting with
Some people can only sleep in total darkness. Others feel safer with a little light on, and would honestly rather keep it. Now that you know what this study found, which side are you on: is the comfort worth it, or is it time to switch it off? Tell me in the comments; I read them.
If this was worth reading, pass it on
- A study of 11,071 people found that sleeping with even a dim light on was linked to thicker, stiffer hearts a few years later. My phone is going face down tonight.
- A heart specialist just said darkness should be treated as a vital sign, as essential to heart health as blood pressure and clean air. I did not expect my bedroom to be a heart-health issue.
- The part that got me: in this heart study, “too much light” started at just 3 lux. That is not a lamp. That is a streetlight around the curtain. Worth knowing.
Tonight, the phone goes face down. The little red light gets a piece of tape. The door gets pulled closed.
It is a small thing. But your heart has been working every minute of every day for as long as you have been alive. Giving it a dark room to rest in may be one of the kindest, cheapest things you can do for it.
