Every day when sunlight hits your eyes, trillions of tiny clocks in your cells are reset. The human body uses light to properly time many processes, ensuring that liver enzymes are produced on time, hair cells divide at the right time, and blood pressure remains at a healthy level. People who do not get their daily dose of light at the right time of day may result in deterioration of health.
But for all its usefulness, researchers are increasingly aware that light also has a dark side. In 2019, one group of researchers discovered association between obesity in women and any level of light exposure during sleep. Another group reported that the light at night was connected to high blood pressure, obesity and diabetes in older people. And in study published in October 2025, the researchers, using light exposure data from fitness monitors worn by nearly 90,000 people and taking readings every minute, found that low ambient light levels at night were associated with a higher risk of heart failure and other cardiovascular problems over about 10 years.
While such studies alone cannot prove that light causes these problems, they add to a growing body of work suggesting that dark nights are necessary for good health.
In the recent study, the team used the largest known database of individual light exposure, part of the UK Biobank data, says Angus Burns, a postdoctoral fellow at Harvard Medical School and an author of the paper. Biobank UK collects information from half a million volunteers, many of whom wore fitness trackers on their wrists for a week. These data strengthened numerous research tying step counting with health results.
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 experienced during the day.
Getting information from binary code was not easy. “It was buried there,” he says. “It's been a long journey.” But when he and his colleague Daniel Windred, now a research fellow at Flinders University in Australia, saw it all before them, they soon realized that while electric lighting had brightened our evenings, there were still clear differences between day and night, with some telling patterns.
Brighter light effects
When the researchers divided 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 the people had very little light at night. However, the other half did not spend that time in complete darkness, and the median value over the six-hour period for people in the top 10% of illuminance was about 100 lux—about the level of a dimly lit hotel hallway. Perhaps they fell asleep with the TV on, or perhaps they woke up late and were still getting ready for the night.
Compared to people whose nights were dark, people whose nights were light were more likely to develop heart disease or have a heart attack over the next ten years or so. The risk was higher the more light they received, and people with very bright nights—the top 10%—had a higher risk of atrial fibrillation and stroke, Windred says. Even when the researchers took into account BMI, prediabetes status, and other health factors, increased risks, which ranged about 30% to 60% higher depending on the condition, still existed. This suggests that light has its own effect.
It's not just that people slept poorly and thus suffered the health consequences of sleep deprivation. “Even after adjusting for how long people slept, light exposure was still a strong and independent predictor of various heart diseases,” says Windred.
“This is consistent with other, smaller studies using personal light sensors,” said Dr. Phyllis Zee, a professor of neuroscience at Northwestern University who studies sleep and circadian rhythms. She helped lead earlier study About 500 older adults who found light at night were associated with an increased risk of obesity, diabetes and hypertension. In another study After examining nearly 700 pregnant women, she and her colleagues found that greater exposure to light before bed was associated with a higher risk of gestational diabetes. There really seems to be something harmful about night light. “The UK Biobank study actually confirms this even in a larger sample,” she says.
The question is why? What exactly does light do?
State of constant alarm
Night light may somehow affect the circadian clock, possibly stopping production melatonina hormone that helps distinguish day from night. Melatonin production may be delayed or stopped even by short flashes of bright light contact with eyes, as studies have shown. The amount of light these people were exposed to might not seem like much. But in the context of human evolution, it may make sense, Burns says. “At night we get light orders of magnitude brighter than the moon or a fire,” he says.
At the same time, during the day, which we mostly spend inside, “we get daylight that is several orders of magnitude lower than what the sun gives us,” Burns says. Researchers have found that very bright days, likely with lots of time spent outside, and very dark nights may protect against heart problems.
But there may be other factors at play besides disrupting the circadian clock. Zee and her colleagues discovered something surprising when young, healthy volunteers spent one night in the laboratory. Some volunteers slept in light levels of around 100 lux, while others slept in light levels as low as 3 lux, which is close to complete darkness. Although heart rates typically drop while we sleep, the volunteers' heart rates remained high in bright light. When the researchers tested the volunteers' metabolisms the next day, they found that the light sleepers' pancreas had to work harder to produce insulin to control their blood sugar levels. “It was like a flare-up,” Zee says. The nervous system, alarmed by the light, seemed to remain ready for action.
Indeed, in previous work, Windred, Burns and colleagues found that the incidence of type 2 diabetes has increased in the UK Biobank, Biobank volunteers had brighter nights, also suggesting a role for metabolism. Windred suggests that light places additional stress on both the cardiovascular system and metabolism when the body is not expecting it, and over time this additional stress leads to damage. There may be ways to mitigate the effects, says Kenji Obayashi, a professor of epidemiology at the Faculty of Medicine at Nara Medical University in Japan who studies light exposure and who was not involved in the study but finds the results intriguing. “It will be important to examine the results of intervention studies that reduce exposure to night light, such as using eye masks or blackout curtains or blinds to block indoor and outdoor light from reaching the retina at night,” he says.
The conclusions that researchers can draw from these studies are currently limited by the available data. Zee's study lasted just one night, and the UK Biobank data only included one week of light exposure. Having data on thousands of people's exposure to light over thousands of nights, as well as longer-term laboratory studies, will help researchers understand the reason for the link between bright nights and poor health.
“Electrical lighting is a complete departure from our biology. In fact, on an evolutionary scale, it's completely new that we get light at night in this way,” Burns says. This has led to situations for which the body is ill-prepared, even if the details are still unclear to scientists. So if you regularly stay up late at night basking in the glow of the television, you may be doing more than just depriving yourself of sleep. “Just go back to ancestral humans and our connection to the sunny day, where our biology evolved,” Burns says. Was ancient man bathed in light at midnight? – Probably not.
