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Sleeping in a room even a little bit of light can hurt a person's health, study shows

AILSA CHANG, HOST:

Turn off the lights for a good night's sleep - common sense. But in our constantly illuminated world, a lot of people don't follow this advice. Researchers are trying to get a better sense of how light at night can hurt our health. NPR's Will Stone reports.

WILL STONE, BYLINE: Maybe it's falling asleep with the TV or the collective glow of electronics or an intrusive streetlight outside the window. Light pollution creeps into our nighttime hours in all kinds of ways, and research shows it's not good for us - in particular for our metabolic health. So Dr. Phyllis Zee and her team at Northwestern University wanted to know - what would the effects be after just one night of sleep? They designed a small study with 20 people. All of them spent the first night sleeping in a mostly dark room. Then, the second night, half of them slept in what Zee describes as moderately well lit.

PHYLLIS ZEE: It's about enough light that you could maybe see your way around, but it's not enough light to really read comfortably.

STONE: As their subjects slept, Zee's team ran tests. They recorded brainwaves, measured heart rate, even drew blood every few hours. Then, after waking up, the participants were given a huge dose of sugar. The researchers found some clear differences. Those sleeping with light had a continuously elevated heart rate compared to their first night in the dark room. And in the morning, they also had more trouble getting their blood sugar into a normal range.

ZEE: And that is what we call a state of insulin resistance.

STONE: What surprised Zee was these physiological changes happened even though their actual sleep wasn't really disturbed. In fact, people generally said they slept fine.

ZEE: It's almost like the brain and the heart and knew that the lights were on, although the individual was sleeping.

STONE: Zee says their study found this relatively small amount of light hitting the eyelids wasn't enough to suppress the hormone melatonin, which is important for sleep, but it appears it was enough to shift the nervous system into a more alert and activated state. Dr. Chris Colwell found this idea intriguing.

CHRIS COLWELL: It wasn't, like, dramatic - as strong as being awake. On the other hand, you know, you don't want that going on when you're trying to get a good night's sleep.

STONE: Colwell studies circadian rhythms and sleep in his lab at UCLA. He wasn't involved in the Northwestern study.

COLWELL: It is a very important example, I guess, of how even relatively dim light exposure in our home environment can be disruptive to our sleep-wake cycle.

STONE: Colwell says scientists have learned a lot in recent decades about the body's light detection system and how inappropriate exposure to light at night can be harmful.

COLWELL: It tends to move animals into a pro-inflammatory state. It can have profound effects on metabolism and cardiovascular function, among others.

STONE: But these sleep studies can be hard to do on humans, and this latest study from Northwestern has its limits. It's small and can't show what would happen if people kept being exposed to this amount of light night after night. Dr. Charles Czeisler directs sleep research at Harvard Medical School and the Brigham and Women's Hospital. He suspects the negative effects would add up.

CHARLES CZEISLER: If you're chronically exposed to this type of thing, that's going to increase the risk of chronic diseases like insulin resistance, diabetes, other cardiometabolic problems.

STONE: For example, one five-year observational study of more than 40,000 women found that having a light or TV on while sleeping was associated with an increased risk of gaining more than 10 pounds. Czeisler says the findings from this Northwestern study underscore a key point.

CZEISLER: People think as long as they fall asleep and are unconscious that it's not having physiological effects, but that's simply not true.

STONE: His advice? Sleep in the dark. If you need a light on to fall asleep, avoid blue light and set a timer so that light or TV eventually shuts off once you're asleep. Will Stone, NPR News.

(SOUNDBITE OF BROKEN SOCIAL SCENE'S "GUILTY CUBICLES") Transcript provided by NPR, Copyright NPR.