When it comes to your sleep setting, the three top tips are to keep it cool, keep it quiet and keep it dark. Anyone who has awoken in a pool of sweat or been startled awake by a noise knows the value of the first two, but light can be a little trickier to understand. Sure, you probably already know that dark versus light environments are better for your sleep. Just as bright light helps you feel more awake in the morning, darkness at night helps you get to sleep and stay asleep.

But sometimes, we fall asleep with the TV on, have light from an outside streetlamp poke through our shades, or use a night light to outline a safe path to the bathroom. In fact, up to 40% of us sleep with some form of light on in our bedrooms. Ever wondered what that light—no matter how dim—is doing to our sleep and our health?

According to the research, it does a lot.

Sleep Research Shows Light at Night Could Hurt

Light (or the absence of it) helps regulate your circadian rhythm, or natural sleep/wake clock. Bright light, especially in the morning, for example, signals your brain to wake up, while darkness at night cues it to power down.

But with our reliance on electricity and devices in the evenings, our lives are becoming increasingly less dark. One 2020 study published in the journal Scientific Reports found that half of homes in the study had lighting that was bright enough to suppress the body’s release of melatonin—considered the “hormone of darkness”—by 50%.

“When it is dark, the body is triggered to release melatonin,” explains Julia Kogan, a health psychologist specializing in sleep and stress. “This is an important hormone that signals to the body that it’s time to fall asleep. Exposure to light at night can interfere with the release of melatonin, since melatonin is released in response to darkness. This can make it hard for people to feel drowsy enough to fall asleep. And light kept on at night through lamps or a TV can also cause disruptions in sleep throughout the night.”

It’s not just the lights you have on inside that you need to worry about. Light streaming in from the outside can also pack a big sleep-disrupting punch.

Research shows that people living in areas with a lot of artificial outdoor nighttime light (think urban areas with lots of streetlights or illuminated business signs) went to bed later, woke up later, slept for less time and felt sleepier during the day.

Even somewhat dim light carries risks. According to Harvard sleep researchers, sleeping in a room with light that’s about twice as bright as a night light can interfere with your circadian rhythm and hinder melatonin secretion.

Recent Sleep Studies Show New Risks

When light exposure affects your sleep, it can also have detrimental effects on your health.

Case in point: Research published in the journal Proceedings of the National Academy of Sciences found that even one night sleeping in a room with moderate light exposure (categorized as enough light to see, but not enough to read) increased nighttime heart rate and insulin sensitivity the next morning, when compared to those who slept in dim light conditions.

According to experts, light triggers a cascade of effects on your body, whether you’re sleeping or not. For starters, light exposure can stimulate your nervous system, and that, in turn, can increase your heart rate. Light at night can affect the way your body regulates glucose, which can increase your risk for insulin resistance (a condition linked to type II diabetes).

Research also shows that sleeping with light can increase your chances of weight gain and obesity. In a study that followed women for about six years, those who slept with the TV or bedroom light on were more likely to gain about 10 pounds during the study period than the participants who slept in darkness, even after adjusting for things like diet, exercise and sleep characteristics. The researchers concluded that light at night may have a direct effect on metabolism.

Ongoing light exposure can shorten the duration of your sleep and worsen the quality of it, which can increase risk factors for numerous issues. Chronic poor sleep, for example, increases your risk of not just heart disease and type II diabetes, but also dementia and certain cancers, such as breast and colon cancer.

Is All Light Bad in the Bedroom?

When it comes to light and your sleep, there are varying degrees of damage to sleep.

At the top of the sleep-disrupting list is blue light, which is the kind emitted from tablets, computer screens, TVs, LED and fluorescent light bulbs, and smartphones.

Researchers had 12 people read eBooks on an iPad (which transmits blue light) for four hours before bedtime for five consecutive nights, and then had them read a print book for five nights. They found that when they read on the iPad, study participants had reduced levels of melatonin, took longer to fall asleep and spent less time in REM sleep than when they read the print book.

Another troublesome light is white light (which contains blue light), says the Centers for Disease Control and Prevention, while yellow light poses minimal risk to your sleep, and red light has been found to have no effect on circadian rhythm.

Intensity of the light also matters. “[Those exposed to] bright light in the evening, as defined by 3,800-6,000 lux [a level of light intensity] from light racks, took longer to fall asleep than people who were exposed to 300 lux of light from a desk lamp,” says Dr. Valerie Cacho, an integrative sleep physician and CEO of Sleephoria. “Additionally, people who live in cities and have high exposure to light pollution report poor-quality sleep.”

What to Know About Sleeping with Red Light

What about those of us who can’t sleep without at least a little light in the room? Recent evidence suggests that red light may be less disruptive to the body's natural sleep-wake cycle than other types of light. Research suggests that this is because red light has a longer wavelength than other colors of light and is not as effective at suppressing melatonin.

While one study found that exposure to red light at night was associated with a decrease in the production of melatonin, the effect was not as strong as with other light colors. However, more research is needed to fully understand the effects of red light on sleep.

Overall, a dark room is preferred; however, if you (or your child) need a night light, try a red lightbulb.

The Darker the Room, the Better the Sleep

Keeping yourself in the dark is key to a better night’s sleep and better health. Here are steps to take:

• Power down your devices. “Turn off your screen 60 minutes before your desired bedtime,” advises Cacho.
• Use glasses or software that help block the blue light emitted from electronic devices. One caveat: “Blue-light-blocking glasses may be helpful, but there’s not a lot of regulation right now, so different glasses will have different extents to which they block blue light. However, there are generally no downsides to using these glasses,” says Kogan.
• Install a dimmer on your bedroom lights or consider using a “smart” bulb or lamp that can be programmed to progressively dim as the evening goes on (some also claim to eliminate blue and green light).
• Invest in blackout curtains.
• Position nightlights as close to the floor as possible, so the light isn’t reaching your eyes. You can even look for one that’s motion-activated, or that has a less sleep-disturbing red or yellow light.
• Remove your TV from the bedroom, or see if it has a sleep timer that you can program to go off before your head hits the pillow. And if you’re only using the TV as background noise to lull yourself to sleep, consider using a sound machine instead.
• Try an eye mask. People in one study reported better sleep quality and had higher levels of nighttime melatonin when they wore the mask versus when they didn’t. A new study published in the journal Sleep found that participants who wore a light-blocking eye mask to bed had improved alertness and memory retention.