Every day, as sunlight streams into your eyes, trillions of tiny clocks in your cells reset. The human body uses light to correctly time myriad processes, ensuring that liver enzymes are made on schedule, hair cells divide at the right time, and blood pressure stays at a healthy level. People who don’t get their daily dose of light at the right time of day can end up with worse health.
But for all its usefulness, researchers are increasingly realizing that light has a dark side. In 2019, one group of researchers found an association between obesity in women and any level of light exposure while sleeping. Another team reported that light at night 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 monitors worn by nearly 90,000 people, taking readings every minute, revealed that low ambient light during the night was linked to a higher risk of heart failure and other cardiovascular problems over about 10 years.
[time-brightcove not-tgx=”true”]
While these types of studies on their own can’t prove that light caused 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, part of the UK Biobank data, says Angus Burns, a research fellow at Harvard Medical School and an author of the paper. The UK Biobank collects information from half a million volunteers, many of whom wore fitness trackers on their wrists for a week. Those data have fueled numerous studies linking step count with health outcomes.
However, the trackers also happened to contain a light sensor. Burns recalls 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 just how much light each person experienced throughout the day.
Getting the information out of the binary code was tricky. “It was buried in there,” he says. “It was a long journey.” But when he and his colleague Daniel Windred, now a researcher at Flinders University in Australia, had it all before them, they soon realized that even though electric lights have made our evenings brighter, there were still clear differences between day and night, with some telling patterns.
The effects of brighter lights
When researchers sorted people into groups based on how much light their trackers picked up between 12 a.m. and 6 a.m., they noticed something interesting. About half of people had very little light exposure at night. However, the other half were not spending that time in total darkness, and the median over the six-hour period, for people in the top 10% of light exposure, was about 100 lux—about the level of a dimly lit hotel hallway. It might be that they had fallen asleep with the TV on, or they might have been awake late and still winding down for the night.
Compared to people with dark nights, people who had brighter nights were more likely to develop heart disease or have a heart attack over the next ten years or so. The risk was greater the more light exposure they had, and the people with the very brightest nights—the top 10%—had higher risks of atrial fibrillation and stroke, says Windred. Even when the researchers took BMI, prediabetes status, and other health factors into account, the elevated risks, which ranged from about 30-60% higher depending on the condition, were still there. This suggests that light has an effect of its own.
It was not merely that people were sleeping poorly and thus suffering from the health effects of sleep deprivation. “Even after adjusting for how much sleep people are getting, the light exposure was still a strong, independent predictor of these various heart diseases,” Windred says.
That tallies with what other, smaller studies with personal light sensors have found, says Dr. Phyllis Zee, a professor of neurology at Northwestern University who studies sleep and circadian rhythms. She helped lead the earlier study of about 500 older adults that found light at night was associated with an elevated risk of obesity, diabetes, and hypertension. In another study of about 700 pregnant women, she and her colleagues found that more light exposure before bedtime was linked to higher risk for gestational diabetes. There does seem to be something damaging about light at night. “The UK Biobank study really confirms that in even a larger sample,” she says.
The question is, why? What exactly is light doing?
A state of constant alert
Light at night may be interfering 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 arrested by even brief flashes of bright light entering the eye, research has shown. The amount of light these people were exposed to might not seem like much. But in the context of how humans evolved, it could be meaningful, says Burns. “We’re getting light at night orders of magnitude brighter than the moon or campfire,” he says.
At the same time, during the day, which we mostly spend inside, “we’re getting daylight exposure that is orders of magnitude lower than what the sun gives us,” Burns says. The researchers found that having very bright days, probably with lots of time spent outside, and very dark nights may protect against heart problems.
But there may be other factors in play, beyond disrupting the circadian clock. Zee and her colleagues uncovered something surprising when they had young, healthy volunteers sleep in the lab for one night. Some volunteers slept in ambient light of about 100 lux and some in only 3 lux, which is close to total darkness. While heart rates usually go down while we’re sleeping, the heart rates of the bright-light volunteers stayed high. When the researchers tested the volunteers’ metabolisms the next day, they found that the brighter light sleepers’ pancreases were having to work harder at making insulin to keep blood sugar in check. “It was almost like being in a heightened state,” Zee says. The nervous system, alerted by the light, seemed to stay ready for action.
Indeed, in previous work, Windred, Burns, and colleagues found that rates of Type 2 diabetes were elevated in the UK Biobank volunteers who had brighter nights, which also points to a role for metabolism. Windred speculates that there is extra stress put on both the cardiovascular system and metabolism by light when the body doesn’t expect it, and over time, that extra stress leads to damage. There might be ways to mitigate the effects, says Kenji Obayashi, a professor of epidemiology at 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 shutters to block indoor and outdoor light from reaching the retina at night,” he says.
The conclusions researchers can draw from these studies so far are limited by the data. Zee’s study was only a single night, and the UK Biobank data include only a single week of light exposure. Having light-exposure data for thousands of individuals over thousands of nights, as well as lengthier lab-based studies, would help researchers get to the bottom of the link between brighter nights and poor health.
“Electric lighting is totally aberrant to our biology. It’s brand new, essentially, on the evolutionary scale, that we have light at night in this way,” says Burns. It has led to situations that the body is ill-equipped for, even if the details are still fuzzy to scientists. So if you find yourself regularly up late at night, basking in the TV’s glow, you might be doing more than just depriving yourself of sleep. “Just take yourself back to an ancestral human and our connection with the solar day, which is where our biology developed,” Burns says. Was an ancestral human bathing in light at midnight? “Probably not.”
Leave a comment
