Turning off the lights and closing the curtains isn't exactly a catchy, new sleep hygiene hack, but this common sense advice is gaining even more scientific credibility.
Many Americans sleep in a room that's punctuated with some form of artificial light — whether it's coming from a TV, a jumble of electronics or an intrusive streetlight.
New research suggests that one night of sleep with just a moderate amount of light may have adverse effects on cardiovascular and metabolic health.
"I was surprised that even this fairly, I would say, small amount of light just getting through the eyes to the brain still had such notable effect," says Dr. Phyllis Zee, senior author of the new study and director of the Center for Circadian and Sleep Medicine at Northwestern University.
The findings tie into a broader body of evidence that indicates being exposed to light at night may be harmful in a variety of ways and could predispose people to chronic diseases.
Physiological effects of light
The small, 20-person study conducted by Zee and her team at Northwestern was designed to measure the physiological effects of 100 lux of artificial light on healthy adults while they were sleeping.
"This is about enough light that you could maybe see your way around, but it's not enough light to really read comfortably," says Zee. For the study, all the 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 overhead).
Meanwhile, the researchers ran tests on the sleepers: they recorded their brainwaves, measured their heart rates and drew their blood every few hours, among other things. In the morning, they'd give both groups a big dose of sugar to see how well their systems responded to the spike.
The results, published in the Proceedings of the National Academy of Sciences this month, show several clear differences between the two groups.
Unlike those who spent both nights in the dark, the group exposed to the light had elevated heart rates throughout the night. They also had increased insulin resistance in the morning, meaning they had more trouble getting their blood sugar into a normal range.
Light can disrupt metabolism
Zee says there are multiple potential ways that being exposed to light at night could disturb our metabolism.
One possibility — supported by research — is that having the light on disrupts the quality of sleep, but surprisingly this study did not find that result while monitoring the people in the lighted room. In fact, the participants generally reported that they thought they slept fine.
The researchers also measured levels of melatonin, a hormone that helps with the timing of circadian rhythms and promotes sleep. Melatonin is typically suppressed during the day and rises at night.
Studies show artificial light at night can suppress melatonin levels, and scientists have found a link between the disruption of melatonin and several diseases, including cancer and diabetes. Though here, too, the study did not find evidence that melatonin levels were lower among the people sleeping with the light on.
"That probably means that the light level that was getting through the eyes was not really bright enough to suppress melatonin," says Zee.
However, Zee and her team believe that this small amount of light was enough to activate the sympathetic arm of the autonomic nervous system — what's responsible for the body's fight or flight response. This is supposed to cool down during sleep as the body moves into a parasympathetic state, when the body's heart rate and respiration decrease.
The changes in cardiovascular function suggest the small amount of light was enough to shift the nervous system to a more activated and alert state.
"It's almost like the brain and the heart knew that the lights were on, although the individual was sleeping," says Zee.
The study is an important example of how even relatively dim light exposure can be disruptive to our sleep-wake cycle, says Dr. Chris Colwell, whose lab at UCLA studies the mechanisms underlying circadian rhythms.
He says the findings makes sense because the autonomic nervous system has a robust daily rhythm.
"There's a lot of coordinated actions that have to occur in order for us to get a good night's sleep and the autonomic nervous system balance regulates that," says Colwell.
This effect on the nervous system wasn't "dramatic" — not as if the people were awake — but Colwell says it's still concerning: "You don't want that going on when you're trying to get a good night's sleep."
Increased risk of chronic illness
The study's findings that metabolic health suffered aren't entirely surprising.
Colwell notes there's already a solid pool of research, as well as large population studies, showing that disrupting circadian rhythms makes it harder to regulate blood glucose levels.
Some of these human studies have used a much brighter intensity of light — and not while people were actually sleeping. And while the findings of this study alone can't predict what would happen in the long term, Colwell suspects the harmful effects would be cumulative: "This was only one night, so imagine if you're living that way constantly?"
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 devices. Cells in the pancreas that secrete insulin are one example. Disrupting the sleep-wake cycle can affect their ability to appropriately secrete insulin, which in turn controls blood sugar.
"That's going to increase the risk of chronic diseases like insulin resistance, diabetes and other cardiometabolic problems," says Dr. Charles Czeisler, chief of the Division of Sleep and Circadian Disorders at the 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 on was associated with a 17% increased risk of gaining 11 lbs over the course of five years.
Czeisler's own research has looked at the metabolic consequences of disruptions in circadian rhythms for longer than just one night.
In a recently published study, he and his colleagues conclude that the negative effects on metabolism observed in their study participants over the course of three weeks were primarily because of disruptions to circadian rhythms — not necessarily because of sleep deficiency.
"When we did not increase their exposure to artificial light at night, we did not see adverse effects of chronic sleep deficiency on glucose metabolism," he says.
This is not to say that sleep deficiency doesn't also have major adverse effects on health — it does — but he says it simply underscores the far-reaching consequences of being exposed to light at nighttime.
"People think that as long as they fall asleep and are unconscious, it's not having physiological effects, but that's simply not true," Czeisler says.
Copyright 2022 NPR. To see more, visit https://www.npr.org.