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Exposure to light during sleep can damage your health

It is far healthier to sleep in a darkened room, finds a new study by scientists at Northwestern Medicine. Close the blinds, draw the curtains and turn off all the lights before bed, they recommend. Exposure to even moderate ambient lighting during nighttime sleep, can increase your heart rate, as well as your insulin resistance the following morning.

It is well known that light stimulates our sympathetic nervous system to become active in the morning when we wake, and this increases heart rate and the speed of metabolic processes. It is now thought that light has a similar effect on us if we are exposed during our sleeping time. And an unbelievable 40 percent of us sleep with some form of light-emitting device in our bedrooms at night – a bedside lamp, the television, or even the main bedroom light. That’s not to mention how many of us have incident light in our bedrooms at night from external sources such as street lights. 

“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 and a Northwestern Medicine physician. “It’s important for people to avoid or minimize the amount of light exposure during sleep.”

The autonomic nervous system has two branches that act with opposite effects. The sympathetic branch is activated during the day and readies us for the tasks and challenges we will face. The parasympathetic branch takes over during sleep and it slows us down in order for the body to repair and restore itself. 

“We showed your heart rate increases when you sleep in a moderately lit room,” said Dr. Daniela Grimaldi, a co-first author and research assistant professor of neurology at Northwestern. “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.” 

The researchers tested the effect on participants’ heart rate of sleeping with 100 lux (moderate) compared to 3 lux (dim) of light over a single night. They found that moderate light exposure caused the body to remain in a more alert state. In this state, the heart rate increases as well as the force with which the heart contracts, and the speed of blood flow through the blood vessels.

“These findings are important particularly for those living in modern societies where exposure to indoor and outdoor nighttime light is increasingly widespread,” noted Dr. Zee. 

In addition, the researchers measured insulin resistance in participants after they spent a night sleeping in a lit room. The participants showed insulin resistance the following morning. Insulin is the hormone that helps keep blood sugar at acceptable levels and insulin resistance refers to the condition where cells in the muscles and liver don’t respond to the presence of insulin. This means that glucose from the blood cannot be stored and the levels of sugar cannot be regulated between meals. In response, even though the pancreas secretes more and more insulin, the blood glucose increases to dangerous levels over time. 

In a previous study, published in the Journal of the American Medical Association, researchers investigated a large population of healthy people who were exposed to light during their sleep time. The study found that these people were more likely to be overweight or obese, explained Dr. Zee. “Now we are showing a mechanism that might be fundamental to explain why this happens. We show it’s affecting your ability to regulate glucose.” 

The participants in the current study weren’t aware of the biological changes in their bodies at night. “But the brain senses it,” said Dr. Grimaldi. “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.”

Exposure to light during the daytime has important health benefits, not least of them being the production of vitamin D in the body. However, it seems that exposure to light during the dark hours carries potential negative consequences for health. 

“In addition to sleep, nutrition and exercise, light exposure during the daytime is an important factor for health, but during the night we show that even modest intensity of light can impair measures of heart and endocrine health,” said Dr. Zee.

According to Dr. Zee, the following tips will help reduce the effects of light while we sleep: 

  • Don’t turn lights on. If you need to have a light on (which older adults may want for safety), make it a dim light that is close to the floor.
  • If you are able to see things really well in your room at night, you probably have too much light for your own good health.
  • Color is important. Amber, red or orange light is less stimulating for the brain. Don’t use white or blue light and keep it far away from the sleeping person.
  • Blackout shades or eye masks are good if you can’t control the outdoor light. Move your bed so the outdoor light isn’t shining on your face.

The results of this study are published today in the journal Proceedings of the National Academy of Sciences

By Alison Bosman, Staff Writer

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