Sleeping in the dark can reduce your risk of heart disease and diabetes, a study suggests.
Exposure to overhead lighting during sleep at night, compared to sleeping in a dimly lit room, harms heart function during sleep and affects how well the body responds to insulin the next morning, researchers found.
They say it is important for people to avoid or minimise the amount of light exposure during sleep, and that if people are able to see things well, it is probably too light.
The study found that, when exposed to more light during sleep, the body went into a state of alert, with the heart rate rising and the body not able to rest properly.
The scientists people should not turn lights on but if they do need to have some light – for example, in the interests of safety for older adults – it should be dim and closer to the floor.
The colour is also important, with amber or a red-orange light less stimulating for the brain.
White or blue light should be kept far away, the experts suggest.
Blackout curtains or eye masks are a good option if outdoor light cannot be controlled.
“The results from this study demonstrate that 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 senior study author Dr Phyllis Zee, chief of sleep medicine at Northwestern University Feinberg School of Medicine in America.
Dr Daniela Grimaldi, a co-lead author and research assistant professor of neurology at Northwestern, said: “We showed your heart rate increases when you sleep in a moderately lit room.
“Even though you are asleep, your autonomic nervous system is activated. That’s bad.
“Usually, your heart rate together with other cardiovascular parameters are lower at night and higher during the day.”
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The study of 20 people found that insulin resistance occurred the morning after people slept in a light room.
This is when cells in muscles, fat and the liver do not respond well to insulin and cannot use glucose from the blood for energy.
To make up for it, the pancreas makes more insulin and, over time, blood sugar goes up.
The people in the study were not aware of the biological changes in their bodies at night.
“But the brain senses it,” Dr Grimaldi said. “It acts like the brain of somebody whose sleep is light and fragmented. The sleep physiology is not resting the way it’s supposed to.”
Researchers tested the effect of sleeping with moderate overhead lighting compared to dim lighting over a single night.
The study shows that moderate light exposure caused the body to go into a higher alert state.
In this state, the heart rate increases, as well as the force with which the heart contracts and the rate of how fast the blood is conducted to your blood vessels for oxygenated flow.
“These findings are important, particularly for those living in modern societies where exposure to indoor and outdoor night-time light is increasingly widespread,” Dr Zee said.
The findings are published in Proceedings of the National Academy of Sciences.
