Young woman lies in bed at night while looking at a cell phone.

Turn off all the lights at night: Your heart will thank you

You’re ready for bed, so you turn off the lights and pull down the shades. Sure, a little light may stream from the sides of the window, or beam from your alarm clock, or TV modem, or cell phone.

No big deal, you say?

Think again. It turns out that even tiny amounts of nighttime light—from any source—may be harmful to your heart. 

One recent study found that older adults ages 63 to 84 who were exposed to even moderate amounts of ambient light during bedtime were more likely to be obese, have diabetes, and have high blood pressure – all risk factors for heart disease – compared to adults who were not exposed to any light during the night. The study, supported by the NHLBI, appeared in the journal SLEEP.

Another study involving adults in their 20s showed that light exposure during sleep can increase insulin resistance, a risk factor for diabetes, the following morning. That study, also funded by the NHLBI, was published in PNAS.

“The link between light at night and cardiovascular disease has been overlooked for a long time. Now, people are beginning to recognize that this is a problem,” said the SLEEP study’s corresponding author Minjee Kim, M.D., an assistant professor of neurology at Northwestern University Feinberg School of Medicine’s Center for Circadian & Sleep Medicine in Chicago.

In her study, the subjects wore a special watch that detects the amount and duration of ambient light while they slept. Although the precise sources emitting the light were unknown to the researchers during the study, Kim suspects that sources as seemingly benign as a streetlight or bathroom light were problematic. “The good news is that it’s a potentially modifiable risk factor for heart disease. People may be able to lower their risk by avoiding or minimizing the amount of light exposure during sleep.”  

The exact mechanism behind this link is unclear, Kim said. Studies suggest that light exposure at night can disrupt the body’s normal circadian rhythm, the 24-hour internal body clock that controls your sleep/wake cycle. This can trigger a cascade of metabolic or biochemical changes that affect glucose and cardiovascular regulation, boosting the risk of heart disease. Kim noted that some research suggests that not getting enough bright light during the day increases one’s sensitivity to light at night and that some people may be genetically predisposed to light sensitivity at night. The specific factors behind the health effects of light at night are the subject of ongoing studies, and Kim said she anticipates more will come.   

“We live in a very different world than two hundred years ago,” Kim said. “There’s so much more light at night than in the past when we just had the moon, stars, and candlelight, and it might be contributing to the current epidemic of cardiovascular and metabolic disease.” Her recommendation: “Don’t wait until stronger evidence comes out. Avoid light exposure at night as much as possible.”  

To reduce the amount of light in the bedroom, researchers recommend turning off lamps, computers, tablets, cell phones, and other light-emitting electronics before you go to bed. Some of those devices, particularly cell phones, emit wavelengths of blue light that can mimic daylight and interrupt sleep. Studies have also linked blue light exposure at night to increased cardiovascular risks, including obesity. As a result, some health experts recommend using a blue-light filter if avoiding the device altogether is not possible.

Another tip: If you have light coming in from the windows, cover it fully or, at a minimum, move your bed so the light isn’t shining in your face. For the safety of older people, researchers do recommend a small, warm-colored nightlight—like red or amber—to reduce the chances of falls at night.

Increasing exposure to natural sunlight during the day is just as critical to protecting your heart and your sleep health as limiting exposure to artificial light, noted Marishka Brown, Ph.D., director of the NHLBI’s National Center on Sleep Disorders Research.

“Getting exposure to sunlight during the day, particular early morning sunlight, is important,” Brown said. “This signal is critical for the regulation of circadian rhythms, which impact many core functions of the body, including blood pressure and metabolism,” Brown said. “This daytime light actually helps you improve your sleep at night”—which, in turn, can have a positive effect on your overall health and wellbeing, she said.

Other ways to get better sleep: going to bed and waking up at the same time daily and reducing caffeine, nicotine, and alcohol intake before bed. If you still have trouble sleeping at night, consult your healthcare provider, Brown said.

The bottom line: “There’s a lot you can do to build a healthy heart,” Kim said. “And keeping your surroundings dark when you turn in for the night is an easy place to start.”