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Turning off the lights and turning off the screen isn’t exactly an interesting, new sleep hygiene hack, but this common sense advice is gaining more scientific credibility.
Many Americans sleep in a room that is punctuated with some kind of artificial light – it’s coming from a TV, an electronics jam or an intruder street light.
New research suggests that sleeping with moderate amounts of light at night may have adverse effects on cardiovascular and metabolic health.
“I was surprised to find that even if a small amount of light enters the brain only through the eyes, it has such a significant effect,” said Dr. Phyllis G., senior author of the new study and circadian and sleep medicine at the Center for Northwestern University.
The results correspond to a wide range of evidence indicating that exposure to light at night can be harmful in a variety of ways and can lead to chronic illness in people.
The physiological effects of light
The small, 20-person study conducted by G and his team in the Northwest was designed to measure the physiological effects of 100 lux artificial light during sleep in healthy adults.
“It’s light enough that you might see around you, but it’s not really light enough to read comfortably,” G says. For the study, all participants spent their first night sleeping in a mostly dark room. The next night, half of them slept in a more illuminated room (the light was placed over their heads).
Meanwhile, researchers conducted experiments on sleeping people: they recorded their brain waves, measured their heart rate and, among other things, drew their blood every few hours. In the morning, they give both groups a large dose of sugar to see how well their systems respond to the spike.
The results, published this month in the Proceedings of the National Academy of Sciences, show several clear differences between the two groups.
Unlike those who have spent both nights in the dark, the team’s exposure to light has increased their heart rate throughout the night. They also increased their insulin resistance in the morning, which meant they had more trouble getting their blood sugar back to normal.
Light can disrupt metabolism
There are several possible ways in which exposure to light at night can interfere with our metabolism, says G.
One possibility – supported by research – is that having light disrupts the quality of sleep, but surprisingly this study did not find that result when observing people in an illuminated room. In fact, participants generally reported that they thought they slept well.
Researchers have also measured levels of melatonin, a hormone that helps circadian rhythms and promotes sleep. Melatonin is usually suppressed during the day and increases at night.
Studies show that artificial light at night can suppress melatonin levels, and scientists have found a link between melatonin disruption and various diseases, including cancer and diabetes. Here, too, the study found no evidence that melatonin levels were lower in people who slept with light on.
“This probably means that the light level passing through the eye was not bright enough to suppress melatonin,” G said.
However, G and his team believe that this small amount of light was enough to activate the sympathetic hand of the autonomic nervous system – what is responsible for the body’s fight or flight response. It is supposed to cool down during sleep because the body goes into a parasympathetic state, when the body’s heart rate and respiration decrease.
Changes in cardiovascular function indicate that a small amount of light was sufficient to move the nervous system to a more active and alert state.
“It’s almost like the brain and the heart knew the lights were on, even though the person was sleeping,” G said.
The study is an important example of how even relatively dim light exposure can disrupt our sleep-wake cycle, says Dr. Chris Colwell, whose lab UCLA studies the underlying mechanisms of circadian rhythm.
He says the results are understandable because the autonomic nervous system has a strong daily rhythm.
“We need to do a lot of coordinated work to get a good night’s sleep and control the balance of the autonomic nervous system,” said Colwell.
The effect on the nervous system was not “dramatic” – not that people are awake – but Colwell said it was still worrying: “You don’t want to be trying to get a good night’s sleep.”
Increases the risk of chronic illness
The results of the study show that the damage to metabolic health is not entirely surprising.
Colwell notes that there is already a strong pool of research, as well as studies of large populations, which show that disrupting circadian rhythms makes it difficult to control blood glucose levels.
Some of these human studies used bright intensities of light – and not when people were actually sleeping. And while the results of this study alone cannot predict what will happen in the long run, Colwell suspects that the detrimental effects will be increasing: “It was only one night, so imagine you’re constantly living like this?”
The body’s “master clock”, called the suprachiasmatic nucleus, is found in the brain, but organs and tissues throughout the body have their own cellular timekeeping device. An example is the cells in the pancreas that secrete insulin. Disrupting the sleep-wake cycle can affect their ability to secrete insulin properly, which regulates blood sugar.
“This will increase the risk of chronic diseases such as insulin resistance, diabetes and other cardiometabolic problems,” said Dr. Charles Chaisler, head of the Department of Sleep and Circadian Disorders at Brigham and Women’s Hospital in Boston and a professor at Harvard Medical. School.
For example, a large observational study of more than 40,000 women found that sleeping with a TV or light increased their risk of gaining 11 pounds by 17% in five years.
Chessler’s own research looked at the metabolic consequences of circadian rhythm disturbances lasting more than just one night.
In a recently published study, he and his colleagues concluded that negative effects on metabolism were observed in their study participants over three weeks, primarily due to circadian rhythm disturbances – not necessarily due to lack of sleep.
“While we did not increase their exposure to artificial light at night, we did not see the adverse effects of chronic sleep deprivation on glucose metabolism,” he says.
It goes without saying that sleep deprivation does not have a major adverse effect on health – it does – but it does say that it only underscores the far-reaching consequences of exposure to light at night.
“People think that as long as they’re asleep and unconscious, it doesn’t have a physiological effect, but that’s just not true,” Chaseler said.