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

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 Digest, Real Simple, Prevention, The Washington Post and The New York Times.