Sleeping in complete darkness isn’t just a preference—it may be essential for your health. New research shows that even small amounts of light at night can disturb your body’s natural rhythm, harm your sleep, and increase long-term health risks.
How Light at Night Disrupts Sleep
The body has a built-in clock called the circadian rhythm. It helps regulate when we sleep and wake. This rhythm depends on natural light and darkness.
Even dim light—like a phone screen or nightlight—can confuse this system. When exposed to light at night, your brain may delay melatonin release. Melatonin is the hormone that helps you fall and stay asleep. Without it, your sleep becomes lighter and more fragmented.
“Sleeping in darkness helps align our internal rhythms with nature,” said Dr. Roger Washington, a sleep-focused physician and director of the Sleep to Live Well Foundation.
Mental Health and Nighttime Light
Sleep and mental health go hand in hand. When you don’t get enough deep sleep, it’s harder to regulate emotions. Anxiety, irritability, and even depression can follow.
A 2024 study of over 13,000 college students in China found that those who used screens at night or slept with the lights on reported more mental health issues, including stress and anxiety. These problems were linked to poor sleep patterns caused by light exposure.
“Artificial light tricks the brain into thinking it’s still daytime,” explained psychologist Leah Kaylor. “This disrupts sleep and leaves your nervous system on high alert.”
Physical Risks of Sleeping With Light
Studies show that exposure to light at night can harm more than just your mood.
A 2023 U.S. study of over 550 adults aged 63 to 84 found that even low levels of nighttime light can disturb sleep patterns and daily rhythms, potentially increasing the risk of obesity, diabetes, and high blood pressure—though not high cholesterol. Even moderate light levels—about 100 lux—can raise your heart rate and interfere with blood sugar control.
Researchers also found that outdoor light at night may raise the risk of coronary heart disease. Over an 11-year period, people living in brighter areas experienced more hospitalizations and deaths related to heart issues.
Links to Obesity and Diabetes
Poor sleep impacts your metabolism. Your body becomes less efficient at processing sugar, leading to weight gain and higher diabetes risk.
A cohort study of 678 seniors found that those exposed to nighttime light of 5 lux or more were three to four times more likely to develop diabetes within 3.5 years. The results indicate that even low-level light exposure at night may greatly increase diabetes risk in older adults.
A large UK study published in The Lancet involving nearly 85,000 adults found that higher nighttime light exposure was linked to an increased risk of developing Type 2 diabetes. Participants wore light sensors for a week, Circadian amplitude and phase were modeled from weekly light data. Incident type 2 diabetes was then recorded (1997 cases; 7.9 ± 1.2 years follow-up). Researchers noted that brighter night light correlated with greater risk—even after adjusting for age, lifestyle, and genetics. Irregular light exposure and disrupted circadian rhythms further raised the risk.
“Even dim light can impair your sleep and mess with glucose regulation,” Dr. Washington said.
Is There a Cancer Risk?
Some research suggests a connection between nighttime light and certain cancers.
One large study found that people in highly lit areas had a 27% higher risk of developing pancreatic cancer. A meta-analysis of 17 studies also linked light exposure at night with an increased risk of breast cancer—especially among premenopausal women with estrogen-sensitive tumors.
Experts believe this may be due to disrupted melatonin levels and poor overnight cell repair, which could make cancer more likely to develop.
What a Healthy Sleep Environment Looks Like
Your bedroom should be as dark as possible—ideally under 1 lux, according to Dr. Washington. That’s when you can’t see your hand in front of your face.
If you must have light, red-spectrum bulbs are the least disruptive. Avoid overhead lights and screens before bed. Use blackout curtains to block outside light. A contoured sleep mask can also help.
“The darker, the better,” said Kaylor. “A calm, dark room helps your brain rest and repair.”
Deep, uninterrupted sleep is essential for your mental clarity, emotional strength, and long-term health. Creating a dark and quiet sleep space isn’t just about comfort—it’s a step toward better living.
