Your Bedroom Probably Isn’t Dark Enough

Every day, as sunlight enters your eyes, billions of little clocks in your cells reset. The human body uses light to properly time myriad processes, ensuring that liver enzymes are produced on schedule, hair cells divide at the right time, and blood pressure remains at a healthy level. People who don’t get their daily dose of light at the right time of day can end up with poorer health.

But for all its usefulness, researchers are increasingly realizing that light has a dark side. In 2019, a group of researchers discovered an association between obesity in women and any level of light exposure during sleep. Another team reported that nighttime light was linked to high blood pressure, obesity and diabetes in older adults. And in a study published in October 2025, researchers drawing on light exposure data from fitness trackers worn by nearly 90,000 people, taking measurements every minute, found that low ambient light at night was linked to a higher risk of heart failure and other cardiovascular problems over about 10 years.

While these types of studies alone cannot prove that light causes these problems, they add to a growing body of work suggesting that good health requires a dark night.

In the recent study, the team used the largest known database of information on personal light exposure, which is part of the UK Biobank data, says Angus Burns, a researcher at Harvard Medical School and author of the paper. The UK biobank is collecting information from half a million volunteers, many of whom wore wrist fitness trackers for a week. This data has fueled numerous studies linking step counts to health outcomes.

However, the trackers also contained a light sensor. Burns remembers discovering this fact and realizing that if he could figure out how to extract the data, he could have a minute-by-minute record of how much light each person received throughout the day.

Extracting information from the binary code was tricky. “He was buried in there,” he said. “It was a long trip.” But when he and his colleague Daniel Windred, now a researcher at Flinders University in Australia, had all this in front of them, they quickly realized that although electric lights made our evenings brighter, there were still distinct differences between day and night, with revealing trends.

Brighter lighting effects

When the researchers divided people into groups based on the amount of light their trackers captured between midnight and 6 a.m., they noticed something interesting. About half of the people had very little exposure to light at night. However, the other half did not spend this time in complete darkness, and the median value over a six-hour period, for people in the highest 10% of light exposure, was around 100 lux, or about the level of a dimly lit hotel corridor. They may have fallen asleep with the TV on, or they may have woken up late and are still getting ready for the night.

Compared to people with dark nights, people with brighter nights were more likely to develop heart disease or suffer a heart attack over the next ten years. The risk was greater the more light they were exposed to, and people with the brightest nights — the wealthiest 10 percent — had higher risks of atrial fibrillation and stroke, Windred says. Even when researchers took into account BMI, prediabetes status and other health factors, the elevated risks, which ranged from about 30 to 60 percent higher depending on the disease, were still there. This suggests that light has an effect of its own.

It’s not just that people sleep poorly and therefore suffer from the effects of lack of sleep on their health. “Even after adjusting for how much sleep people got, light exposure remained a strong and independent predictor of these various heart diseases,” says Windred.

That’s consistent with what other, smaller studies have found with personal light sensors, says Dr. Phyllis Zee, a professor of neurology at Northwestern University who studies sleep and circadian rhythms. She helped lead an earlier study of about 500 older adults that found nighttime light was associated with a high risk of obesity, diabetes and hypertension. In another study of about 700 pregnant women, she and her colleagues found that increased exposure to light before bed was linked to a higher risk of gestational diabetes. There seems to be something damaging about the night light. “The UK Biobank study really confirms this in an even larger sample,” she says.

The question is, why? What exactly does light do?

A constant state of alert

Nighttime light may interfere with the circadian clock in some way, perhaps by stopping the production of melatonin, a hormone that helps differentiate day from night. Melatonin production can be delayed or stopped even by brief flashes of bright light entering the eye, research has shown. The amount of light these people were exposed to may seem minimal. But in the context of human evolution, it might make sense, Burns says. “At night, the light is much brighter than that of the moon or a campfire,” he says.

At the same time, during the day, which we spend mostly indoors, “we get exposure to daylight that is an order of magnitude less than what the sun gives us,” says Burns. Researchers have found that very bright days, likely with lots of time spent outdoors, and very dark nights may protect against heart problems.

But there may be other factors at play, beyond circadian clock disruption. Zee and his colleagues discovered something surprising when they had healthy young volunteers sleep in the lab overnight. Some volunteers slept in ambient light of around 100 lux and others in only 3 lux, which is close to total darkness. While heart rates typically decrease while we sleep, bright light volunteers’ heart rates remained elevated. When the researchers tested the volunteers’ metabolism the next day, they found that the pancreases of light, bright sleepers had to work harder to produce insulin to control blood sugar levels. “It was almost like being in an acute state,” Zee says. The nervous system, alerted by the light, seemed to remain ready to act.

Indeed, in previous work, Windred, Burns and their colleagues found that rates of type 2 diabetes were elevated among British Biobank volunteers who had brighter nights, also indicating a role for metabolism. Windred speculates that light puts extra stress on the cardiovascular system and metabolism when the body doesn’t expect it, and that over time this extra stress leads to damage. There might be ways to mitigate the effects, says Kenji Obayashi, a professor of epidemiology at the Nara Medical University School of Medicine in Japan who studies light exposure, who was not involved in the study but finds the results intriguing. “It will be important to examine the results of interventional studies that reduce nighttime light exposure, such as using eye masks, blackout curtains, or blinds to block indoor and outdoor light from reaching the retina at night,” he says.