The study of 20 healthy adults found that just one night of sleeping with the lights on spurred changes in people's functioning: Their heart rates stayed higher during sleep compared to a night with lights off. And, by the next morning, they were churning out more insulin — a hormone that regulates blood sugar.
The effects were not dramatic. But it's plausible that small effects, night after night, could ultimately affect a person's health, said senior researcher Dr. Phyllis Zee.
"This study doesn't prove that, and we need more research to look at chronicity," said Zee, chief of sleep medicine at Northwestern University Feinberg School of Medicine, in Chicago.
But in the meantime, she said, there's little downside to dimming the lights before going to sleep.
"I don't think people need to sleep in total darkness," Zee said. Even our technology-free ancestors were exposed to moonlight, she noted.
So if you need a nightlight for safety, Zee said, that's fine. Just make it less bright, and place it closer to the floor. A light on the red/amber end of the spectrum is better than white or blue light, Zee added, because it is less stimulating for the brain.
It's well known that people need exposure to sunlight during the day, and darkness at night, to keep the body's circadian rhythms running optimally. Those rhythms, which are like 24-hour internal clocks, help regulate processes throughout the body — including sleep, metabolism and hormone release.
But modern humans are exposed to all kinds of artificial light at night, and research has been pointing to the pitfalls. Exposure to blue light from glowing devices may be especially problematic: It suppresses the body's release of the sleep hormone melatonin, making us feel more alert when we should be unwinding.
There has been less research, Zee said, on the potential effects of artificial light during sleep.
So she and her colleagues recruited 20 healthy young adults for a sleep-lab study. Ten spent two nights in a row sleeping with only a dim nightlight. The other 10 had dim lighting for one night, but on the second night slept with overhead lights on — enough to bathe the room in a moderate amount of light, Zee said.
On average, that lights-on condition spurred a small increase in volunteers' heart rates, plus a decrease in their heart rate variability (a bad thing), the researchers found. It also boosted their bodies' resistance to the effects of insulin the next morning. Insulin resistance, if sustained, can be a precursor to type 2 diabetes.
"This provides a biological plausibility that nighttime light exposure could potentially increase the risk for diabetes and other cardiovascular outcomes," said Dr. Kannan Ramar, immediate past president of the American Academy of Sleep Medicine's board of directors.
The study does not prove that, however, stressed Ramar, who was not involved in the work. But he agreed that keeping the bedroom lights dim — and TVs and phones away from the bed — is wise.
It's not that the study volunteers were disturbed by the lights. "They thought they slept pretty well," Zee said.
Nor did the light disturb people's melatonin levels. Instead, Zee said, the effects on heart rate and insulin suggest the light is activating the sympathetic nervous system, which keeps us alert and is generally dialed up during the day.
The study — published online March 14 in theProceedings of the National Academy of Sciences — is not the first to suggest harm from sleeping with the lights on.
A 2019 study of U.S. women found that those who slept with a light or TV on tended to gain more weight over time, and were more likely to become obese, compared to lights-off sleepers.
But there's been little in the way of lab research to search for potential mechanisms, said Matt Lehrer, an assistant professor at the University of Pittsburgh who studies sleep and circadian rhythms.
He called the new study a "good step forward."
Future research, Lehrer said, could include people who already have insulin resistance and elevated blood sugar, to see if lights-on (or TV-on) sleeping makes the issue worse.
Compared with other lifestyle factors, like diet and exercise, less is known about the health effects of light exposure, Lehrer pointed out. But, he said, people should be aware that it matters.
Zee agreed, and noted that getting sunlight during the day is just as important as limiting artificial light at night.
The Sleep Foundation has more on light and sleep.
SOURCES: Phyllis Zee, MD, PhD, chief, sleep medicine, Northwestern University Feinberg School of Medicine, Chicago; Matt Lehrer, PhD, assistant professor, psychiatry, University of Pittsburgh; Kannan Ramar, MD, immediate past president, American Academy of Sleep Medicine, Darien, Ill., and professor, division of pulmonary and critical care medicine, Mayo Clinic, Rochester, Minn.; Proceedings of the National Academy of Sciences, March 14, 2022, online