Light at Night Spoils Sleep and Health

You won’t realize it’s happening, but even moderate light robs you of deep sleep and raises your heart rate

Photo: Unsplash / Shane

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…