Sleeping with the light on may be harmful to you

Sleep mask. Photo: Ela Dutt

From the streetlights outside our bedrooms to the lamps and devices inside, sleeping with some amount of light has become a way of life for many.

That may not be such a bright idea. Research suggests that sleeping in a moderately lit room could affect metabolic and cardiovascular health compared with snoozing in a room with dimmer light.

In a study published in PNAS, researchers at Northwestern University had two groups of 10 young adults sleep in differently lit rooms. One group slept in rooms with dim light for two nights; the other slept one night in a room with dim night and the next in a room with moderate overhead light – about the equivalent of an overcast day. Participants wore heart monitors at night. In the morning, they did a variety of glucose tests.

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Both groups got the same amount of sleep but their bodies experienced very different nights. Both groups responded well to insulin the first night, when they both slept in dim lighting.

On the second night, however, the group sleeping in brighter lighting didn’t respond as well to insulin. The dim light sleepers’ insulin resistance scores fell about 4% on the second night, while the bright sleepers’ rose about 15%. Their heart rates were faster on the bright night, too.

The heightened heart rate and other measures led the researchers to conclude that light activates the sympathetic nervous system, which usually dominates bodily functions during the day.

“Just a single night of exposure to moderate room lighting during sleep can impair glucose and cardiovascular regulation, which are risk factors for heart disease, diabetes and metabolic syndrome,” said Phyllis Zee, chief of sleep medicine at Northwestern University Feinberg School of Medicine and a Northwestern Medicine physician who led the study, in a news release. “It’s important for people to avoid or minimize the amount of light exposure during sleep.”

The study was small and only tracked subjects for a limited time. More investigation is needed, the researchers write, but light exposure during sleep “could have implications for those living in modern societies where indoor and outdoor nighttime light exposure is increasingly widespread.”

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