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Even a Small Amount of Light While You Sleep May Harm Your Health

Light exposure while you sleep leads to an elevated heart rate and higher insulin resistance, study finds

woman asleep with a light on

Enes Evren / Getty Images

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You’ve probably heard that keeping your bedroom dark can help improve the quality of your sleep. But new research reveals that eliminating artificial light during the night — from a TV set or radio clock or streetlight — could also have a positive impact on your heart and blood sugar levels.

In an intriguing new study, Northwestern University researchers found that adults exposed to even a moderate amount of light while they slept experienced higher heart rates compared to when they slept in a very dark room. They also had more insulin resistance in the morning, which meant their bodies had to work harder to regulate their blood sugar, the study showed.

Surprisingly, the study subjects who were exposed to the light didn’t report any trouble sleeping.

“The upshot is, it appears that light during sleep is affecting you, even if you’re not aware of it,” says study coauthor Phyllis Zee, M.D., director of the Center for Circadian and Sleep Medicine at Northwestern University Feinberg School of Medicine. “Your subconscious is aware that there’s light and there’s something going on, and it’s keeping you a little bit on watch. Your fight-or-flight system is more activated.”


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Elevated heart rate and insulin resistance are risk factors for heart disease and early death. The study is important because it highlights a factor you can control that could improve your health and lower your long-term health risk, Zee says.

Effects of light exposure

Previous studies have linked light exposure at night to a variety of health problems, including an increased risk of depression, type 2 diabetes and some types of cancer. In addition, a large study of 43,722 women published in 2019 found that those who slept with artificial light had a higher risk of gaining weight and being obese than those who slept in a dark room. 

In light of that research, Zee says she wanted to design a controlled study to examine what happens to your body physiologically when you are exposed to light while you sleep.

She recruited a group of 20 young adults to spend two nights in her sleep lab and divided them into two groups. Both groups slept in a dark room on the first night. On the second night, the control group again slept in the dark room, but the other group spent the night in a room with moderate light exposure — about equal to that of a streetlight outside or a hallway light left on.

While the control group showed little change from the first night, the group exposed to light had higher heart rates and a significant increase in their insulin resistance levels compared to the night they slept in a darker environment.

“What’s new and novel about our work is that it’s giving us a clue about the mechanism — how light exposure during sleep could lead to metabolic disorders,” Zee says.

Eyelids aren’t designed to filter out light

Although the study was conducted on young people, Zee says her hunch is that the effect on older adults may be even more prominent.

“That’s a population already at risk for insulin resistance,” she says. “It will be very interesting to do these types of studies with older adults because many institutions, like nursing homes, keep the lights on for safety reasons.”

Kristin Daley, a psychologist and sleep medicine expert who chairs the clinical practice committee for the Society of Behavioral Sleep Medicine, says the study offers more evidence that it’s best to sleep in complete darkness if possible — a contention she and other sleep clinicians have been making for years.

“The problem is, our eyelids are not designed to filter out light,” she says. “We all have receptors in back of our retinas that pick up light even when our eyes are closed.”

Your bedroom may not be as dark as you think

Even if you believe your room is dark, you may be exposed to more light than you realize. Glowing electronic devices, like cable boxes, USB chargers and even your alarm clock, can light up a whole room, Daley says.

Daley suggests conducting this test in your bedroom at night: Close the door, turn off the lights, let your eyes adjust to the darkness for a minute or so and hold out your hand in front of you. 

“If you can still see your hand 24 inches away from face, you have too much ambient light,” Daley says. “I have all my patients do this, and it’s funny how many come back and say, ‘Holy cow! I had so much light in my room.’ Their perception was that it was really dark.”

Block LED lights on electronic devices with tape or by covering them, and turn your alarm clock around, Daley suggests. If you live in an urban area, you may need to get room-darkening curtains or shades to fully block the shine from streetlights outside.

What if you get up during the night?

Many older adults keep a light on at night because they have to get up and use the bathroom, and they want to be able to see where they’re going to prevent falls.

If that’s the case, Daley recommends keeping a flashlight on your bedside table and using it to light the way if you visit the bathroom. “It trains the light away from your eye [as opposed to a nightlight, which] directs the light toward your eye,” she explains.

If you prefer to use a nightlight, Zee suggests plugging it in as close to the floor if possible. You may also want to cover it with a red light filter, since studies show red or orange light is less likely than white and blue light to affect brain activity.

“I’m not saying you have to sleep in total darkness, because I understand that may not be possible for some people,” Zee says. “The goal is to minimize light exposure and yet create a safe environment.”

Michelle Crouch is a contributing writer who has covered health and personal finance for some of the nation’s top consumer publications. Her work has appeared in Reader’s DigestReal Simple, Prevention, The Washington Post and The New York Times.

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