Turning off the lights and closing curtains aren’t exactly a new sleep hack, but this common-sense advice now has more scientific credibility.
Sourced from NPR by Will Stone | April 1, 2022
If you are like most Americans (or New Yorkers, ahem) you sleep in a room punctuated with some form of artificial light — think TVs, devices and electronics (…if that’s the case, what’re ya doin’?) or intrusive streetlights. Did you know that new research suggests that just one night of sleep with 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.
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The findings lead to a broader body of evidence that suggests exposure to light at night may be harmful in a variety of ways and can even predispose people to chronic diseases.
Physiological effects of light
Zee and her team at Northwestern conducted a small, 20-person study designed to measure the physiological effects of 100 lux of artificial light on healthy, sleeping adults.
"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).
Researchers ran tests on the sleepers by recording their brainwaves, measuring heart rates, and drawing their blood every few hours, among other things. In the morning, both groups were given 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, show several clear differences between the two groups.
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 nighttime light exposure can disturb our metabolism.
One possibility 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.
“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.
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This effect on the nervous system wasn’t “dramatic” — not as if the people were awake — but it’s still concerning: You don’t want that going on when you’re trying to get a good night’s sleep.
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.
Increased risk of chronic illness
That metabolic health suffered isn'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” ”
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.
