Light at Night Spoils Sleep and Health
Once upon a time, humans slept in total darkness, save perhaps some bright moonlight or a soothing fire at the cave entrance. After millions of years without light switches, we’re evolutionarily programmed to rely on darkness for a good night’s sleep. So it’s no surprise that artificial light in the evening and throughout the night is bad for both sleep quality and duration, as studies have shown. Even full moons mess with human sleep.
A new study reveals that during sleep, the brain detects light that passes through our eyelids, kicking the body’s nervous system into a higher state of alert. That elevates the heart rate, reduces sleep quality, and raises risks for heart disease and diabetes.
“Even though you are asleep, your autonomic nervous system is activated,” says Daniela Grimaldi, MD, an assistant professor of neurology at Northwestern University and co-author of the research paper. “That’s bad.”
You won’t realize what’s happening
The study, done in a sleep lab, included 20 participants ages 19 to 36, none of whom had any sleep disorders or serious health issues. One group slept a night in near darkness followed by a night with a light illuminating the room sufficiently to walk around safely but not bright enough to read by, Grimaldi explains in an email. People in a control group slept in near-darkness both nights.
The sleepers’ heart rates, brain waves, and other vital sleep signs were monitored through the night. Each morning, levels of insulin and glucose were checked.
Compared to those who slept in darkness, the heart rates of participants who slept with the light on were 5% to 20% higher, continuously during the night, and they spent less time in the most restorative stages of deep sleep. In the morning, these individuals had on average 25% higher levels of insulin, a sign of insulin resistance, when the body struggles to convert glucose into energy — a hallmark of Type 2 diabetes.
When questioned the next morning, those who slept with the light on weren’t aware of the negative sleep patterns they’d experienced.
“But the brain senses it,” Grimaldi says. “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.”
“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,” says the study’s senior author, Phyllis Zee, MD, a physician and chief of sleep medicine at Northwestern University’s Feinberg School of Medicine. “It’s important for people to avoid or minimize the amount of light exposure during sleep.”
The findings are detailed in the Proceedings of the National Academy of Sciences.
Light is key to our body’s internal clock, creating a circadian rhythm that governs the sleep-wake cycle. Darkness triggers the production of melatonin, which helps prepare the mind and body for sleep. Outdoor light — which even on a cloudy day is brighter than common home or office lighting — suppresses melatonin, keeping us awake and alert.
Other research has shown that too much light in the evening from indoor lighting, phones, and other screens can delay the onset of sleep, and too little natural outdoor light during the day can have a similar effect. Any outdoor light that shines into a bedroom can hamper sleep, too, studies have found.
The effects of artificial light aren’t dissimilar from those of our favorite natural night light.
During full moons, people tend to get less deep sleep, according to a 2013 study. And in the days leading up to a full moon, people all over the world — those with electricity and those without — go to bed on average a bit later and sleep a little less than at other times of the lunar cycle, a 2021 study published in the journal Science Advances found.
“In general, artificial light disrupts our innate circadian clocks in specific ways: It makes us go to sleep later in the evening; it makes us sleep less,” says Horacio de la Iglesia, PhD, a professor of biology at the University of Washington and leader of the 2021 study.
How bright is too bright?
Though total darkness may be optimal for slumber, some people prefer a bit of light for safety’s sake. Though the new study didn’t test varying light levels, it’s possible, though not certain, that even brighter light might have a greater negative effect, and lighting dimmer than what was tested could have a less-pronounced but still measurable effect, Zee tells me.
There are plenty of other obstacles to sleep, from lack of exercise to too much drinking (check out my top sleep tips here). Meanwhile, Zee offers these suggestions regarding light:
- If you use a night light, make it dim and put it near the floor.
- Avoid bright night lights (the white or bluish-white ones) and instead choose warmer colors — red, orange, or amber — that don’t stimulate the brain as much.
- Turn off annoying outdoor lights if possible, or use blackout shades, or maybe move your bed so outdoor lights don’t shine directly in your face.
- Or try eye masks.
Roughly 5% to 9% of light passes through your closed eyelids. But how can you know if your bedroom is too bright when your eyes are closed? Here is Zee’s simple test, to conduct with your eyes open:
“If you’re able to see things really well, it’s probably too light.”
Thanks to elemental