How often do you reach for your phone before bed and remember that blue light can disrupt your sleep, but you scroll on anyway? Yeah, us too.
Over the past few years, separate research has linked nighttime light exposure to, yes, sleep disruption, but also negative effects on cardiac, metabolic, cognitive, and mental health. It’s also linked to higher cancer risk.
You’d probably guess it’s about circadian rhythm, and you’d be right. But there’s more to it than that. Throwing off our internal clocks produces a cascade of effects across systems: Shifts in hormonal balance, organ function, inflammation levels, and more.
“One of the things that we’ve started to understand more is that our circadian rhythms are closely linked to our overall health,” said Jennifer Martin, PhD, sleep researcher and professor of medicine at the David Geffen School of Medicine at UCLA. “Our digestion has a circadian rhythm. Even our cells dividing have a circadian rhythm.”
How Nighttime Light Became Another Silent Killer
Blame our tech-driven world, which has given us convenience food, long periods of sitting, and now too much light exposure — all contributing to poor health outcomes. Modern lights are brighter, turn on earlier, and shut off later — if at all.
Let’s start with circadian rhythm. “Light is the most important time cue for the body clock,” said Frank A.J.L. Scheer, PhD, professor of medicine at Harvard Medical School and professor of medicine at Brigham and Women’s Hospital, Boston. It keeps the body clock in sync with our 24-hour day. Light itself has acute effects, such as the ability to shift melatonin, spike cortisol, and even alter our reproductive hormones.
There’s more, said Kristen Knutson, PhD, associate professor of neurology and preventive medicine at Feinberg School of Medicine, Northwestern Medicine, Chicago. The circadian system helps regulate numerous processes in our body, including cardiovascular function and blood pressure. When we confuse our internal clock, “there are health consequences,” she said.
Then there’s the strong association between nighttime light exposure and poor quality sleep, the latter of which in itself is linked to everything from obesity to diabetes, cardiovascular disease, accelerated brain aging, and even early mortality.
Indeed, accumulating research now suggests nighttime light exposure can take a toll on nearly every aspect of our health.
Cardiovascular Health
A newly published cohort study in JAMA Network Open followed nearly 89,000 adults over 9.5 years, using wrist sensors to measure personal light exposure at night. When compared with participants who were exposed to darker nights, those with the brightest nighttime light exposure showed significantly higher risks of developing a range of cardiovascular conditions, including coronary artery disease, myocardial infarction, heart failure, atrial fibrillation, and stroke — even after adjustment for multiple cardiovascular risk factors like age, physical activity, smoking, and sleep habits. The researchers found that exposure to lights after sundown had the potential to raise cardiovascular risk by up to 50%.
While researchers are still uncovering the mechanisms behind light-related heart disruption, Martin said it may be due to inflammation or the fact that disrupting the circadian rhythm may deprive our cardiovascular system of a period of rest that it should be getting every night. “It may be that our heart isn’t getting a break like it should, and so that creates additional stress, and that stress leads to things like heart attack, stroke, and atrial fibrillation,” she said.
Metabolic Health
Research has found that exposure to lights at night can affect how your body regulates blood sugar. A 2022 study by Northwestern University exposed healthy adults to moderate light in their rooms during sleep. While the total duration of sleep did not significantly change, participants who were sleeping in the room with light experienced increased activation of their sympathetic nervous system — the body’s “fight-or-flight” response — and had elevated heart rates during the night. “This correlated with next-morning glucose control, so the blood sugar regulation the next morning was negatively impacted by the light exposure the night before,” said Scheer. These findings suggest that exposure to light at night may disrupt metabolic health through more than just sleep deprivation.
Additional research found that people who encountered nighttime light were around 19% more likely to have overweight or obesity, nearly 86% more likely to have hypertension, and about 21% more likely to have diabetes than those with the lowest light exposure. And a 2025 analysis found that each 10-lux increase (a lux is a unit of brightness) in nighttime light was associated with a 30% greater risk of developing type 2 diabetes.
Mental and Brain Health
Nighttime light exposure can also take a toll on our mental health, not only making us tired but also affecting our mood as well. A study published in Nature Mental Health performed the largest to date cross-sectional analysis of light, sleep, physical activity, and mental health. The researchers examined 86,772 older adults and found that those with greater nighttime light exposure had significantly higher risks for major depressive disorder, generalized anxiety disorder, posttraumatic stress disorder, psychosis, bipolar disorder, and self-harm behavior compared with those with darker nights. It’s also been tied to hyperactivity, abnormal sociability, and learning and memory deficits.
And some researchers hypothesize that nighttime light exposure can raise inflammation, which may lead to an increase in brain plaques that are present in the brains of patients with Alzheimer’s Disease, potentially leading to an increased risk for dementia.
Poor sleep quality could be a major culprit here — but so could some of the previously-mentioned side effects like elevated fight-or-flight engagement, stress hormones, and heart rate.
Another risk factor we didn’t have 25 years ago: “Screens are everywhere,” said Scheer. “We have screens from the time we open our eyes till we close them.” That means more time working, but also more time reading and watching things that may stress or upset us.
Cancer
This could be why studies like a recent review by the National Toxicology Program have associated persistent circadian disruption, such as that experienced by frequent night shift workers, to an increased cancer risk.
“Prostate cancer in men is linked to light at night, and breast cancer of women, and then colorectal cancer,” said Martin Moore-Ede, MD, PhD, director of the Circadian Light Research Center in Stoneham, Massachusetts. “Those cancers are actually increasing more because one of the big problems that’s occurred in the last 10 years is we’ve replaced all our lights with blue-rich LEDs,” he said, noting that his research has shown that LED lights in particular can be especially circadian disruptive.
Helpful Ways to Reduce Nighttime Light
Adopt a healthy “light diet.” The goal is to master the timing of light. “Light can help us have a good, strong, robust circadian rhythm, or it can get in the way,” said Moore-Ede. As early as you can, “get outside for at least half an hour and get exposed to the full gamut of wavelengths of light,” he said. This starts synchronizing your circadian clock. It’s also important to get light exposure during the day. “Move your desk near a window or take breaks outside,” he said.
Dim after dark. “From the time you go to bed at night until the time that you get up in the morning, that’s a really important time to maintain darkness as much as you can,” said Martin. “If you’re exposed to a lot of bright light just before bedtime, your brain just thinks the day hasn’t ended yet.” She recommends not looking at your phone or TV a minimum of 15 minutes before bed, ideally stopping hours before if you can.
Block more than blue light at night. While research has popularized the fact that blue light exposure at night can contribute to throwing off one’s circadian rhythm, it’s not the only light to avoid. “Blue is not the problem; it’s the time of day you see blue,” said Moore-Ede. “You want to see blue during the daytime, so the worst thing you could do is wear blue blocking glasses during the daytime.” You’re better off simply dimming lights as the sun goes down.
Make your sleep space sacred. Studies show that even a small amount of light exposure at night can harm health. “I think the most important thing is to think about the place where you sleep as a bit of a sanctuary and set it up in a way that keeps the environment dark at night,” said Martin. Consider blackout curtains and if you have to get up in the night to go to the bathroom or get a glass of water, turn on as little light as possible, said Martin, who stresses that the brighter the light, the more of an effect it has on our melatonin production.
“Having a light that turns on near the floor if you have to get up a lot at night is a great idea,” she said. “Turning on all the lights in your kitchen when you get up for a glass of water is not.”
