One lit night raised heart rate and insulin resistance, while darkness costs little

Key takeaways

• In a small Northwestern University experiment, volunteers who slept one night under 100 lux of overhead light had faster heart rates that night and insulin resistance the next morning, unlike those kept in dim light¹².
• In a study of more than 43,000 women, sleeping with a light or TV on was linked to a 17% higher chance of gaining about 11 pounds or more, while a small nightlight was not³.
• Large wrist-sensor studies tie the brightest nights to higher risk of type 2 diabetes and heart failure, but they show links, not proof of cause⁵⁶.
• An eye mask typically costs $10 to $20, and in a Cardiff University study it gave young adults a small boost in next-day memory and reaction time⁷⁸.
• Older adults need both darkness and a safe route to the bathroom, and dim, low, amber path lights can provide both¹¹⁰¹⁵.

At Northwestern University, sleep researchers ran a simple test. Volunteers slept one night under overhead room light of 100 lux, a moderate level. A comparison group slept in dim light under 3 lux¹².

The sleepers didn’t notice anything wrong. Their bodies did. During the lit night, their heart rates rose. The next morning, they showed insulin resistance, meaning their cells responded less well to insulin, the hormone that moves sugar out of the blood¹.

Up to 40% of people sleep with a bedside lamp or another bedroom light on, according to Northwestern¹. Senior author Phyllis Zee offered a quick test for your own bedroom: “If you’re able to see things really well, it’s probably too light”¹.

Darkness, in other words, is more than the absence of light. It’s a signal your body keeps reading all night.

This article covers what the research shows, how to darken a bedroom cheaply, and how older adults can light a safe path without flooding the room.

Darkness is a signal your body waits for

Your brain releases the hormone melatonin from the small pineal gland in response to darkness⁷. Light at night suppresses melatonin and can change how you sleep⁷.

Light can matter even while you sleep. Writing for Harvard Health Publishing, sleep psychologist Eric Zhou points to research in which adults who slept beside a nightlight had shallower sleep and woke more often. Outdoor light at night, such as from street lamps, has been linked to less sleep⁷.

The Northwestern study points to a second route. Moderate light during sleep switched on the sympathetic nervous system, the body’s “fight or flight” branch¹. That raised heart rate and made each heartbeat more forceful¹. The sleepers’ brains registered light they were never aware of¹.

How your body clock sets its timing, and what screens do before bed, are covered in our articles on circadian rhythms and screen habits. This article is about the hours you’re asleep.

One night of room light changed the next morning

The Northwestern experiment appeared in 2022 in the Proceedings of the National Academy of Sciences. According to a summary from Koç University in Turkey, 20 volunteers were randomly assigned to two groups². One group slept two nights in dim light. The other slept one dim night, then one night with overhead lights at 100 lux².

During the lit night, heart rates rose and heart rate variability, the natural beat-to-beat change in your pulse, fell². The morning after, the light group showed insulin resistance¹².

Zee’s summary: a single lit night was enough to throw off the body’s handling of blood sugar and its control of the heart and blood vessels¹.

Keep the size of this study in mind. It covered one night, in a lab, in a small group of volunteers¹². It shows the body responds quickly to light during sleep. It doesn’t show that a bedside lamp causes diabetes.

Big studies tie bright nights to weight gain, diabetes and heart disease

The National Institutes of Health asked a related question in women. Its Sister Study followed more than 43,000 women aged 35 to 74 for an average of 5.7 years³.

At the start, women reported how they slept: with no light, a small nightlight, light coming from outside the room, or a light or TV on in the room³.

Women who slept with a light or TV on were 17% more likely to gain about 11 pounds or more during follow-up³. Light from outside the room had a weaker link. A small nightlight showed no link to extra weight gain³.

The researchers adjusted for many other explanations, including calories, nighttime snacking, activity, socioeconomic status and sleep quality, and the link held³. They also said they couldn’t rule out every other factor³. Scientists call this confounding: a hidden third factor could drive both the light and the weight.

Northwestern turned to older adults in a 2022 study in the journal SLEEP. Researchers fitted 552 people aged 63 to 84 with wrist devices that recorded light for a week⁴. Fewer than half got a steady five-hour stretch of complete darkness each day⁴.

People exposed to any light during sleep were more likely to have obesity, high blood pressure and diabetes⁴. But the study was a single snapshot in time. The authors noted the arrow could point backward: someone with diabetes-related numbness in the feet might keep a nightlight on to avoid falling⁴.

The biggest datasets come from the UK Biobank, a large British health study. A team led by researchers at Flinders University in Australia analyzed light sensors that volunteers wore on the wrist for a week:

• Among 84,790 people followed for about eight years, those with the brightest nights had a 53% higher risk of type 2 diabetes than those whose nights fell in the darkest half. Risk rose step by step with brightness⁵.
• Among 88,905 people followed for up to 9.5 years, the brightest nights went with a 56% higher risk of heart failure and a 47% higher risk of heart attack⁶. Some heart links were stronger in women and in younger participants⁶.

Both studies adjusted for lifestyle, socioeconomic status and genetic risk⁵⁶. Earlier studies had used satellite maps of outdoor light. Those also tied brighter nights to diabetes, but more weakly, and a satellite can’t see the lamp in your bedroom⁵.

One caution from us: a wrist sensor can’t tell whether light fell on a sleeping person or on someone awake at 2 a.m. People who are up at night for work, worry or illness may have both more light and more health risk.

Eye masks: cheap, with a small payoff in one study

The simplest way to make your night dark is to cover your eyes. Researchers at Cardiff University in Wales tested that idea in a study published in SLEEP in 2023⁸.

In the first experiment, 94 adults aged 18 to 35 slept for a week with an eye mask and for a week without light blocking⁸. In later rounds, the comparison mask had holes over the eyes, so it felt similar but let light through⁹. On days six and seven, everyone took memory and alertness tests⁸.

With the real mask, people learned word pairs better and reacted slightly faster⁸⁹. A second experiment with 35 adults repeated the memory benefit, and people who got more slow-wave sleep, the deepest stage, gained more⁸.

The gains were small. Statistically, the memory effect was about one-fifth of a standard deviation, a real but modest edge⁹. Self-reported sleep length didn’t change⁹. And everyone tested was a healthy young adult⁷.

At a typical $10 to $20, a mask makes a fair personal experiment⁷. If noise wakes you too, our upcoming article on noise covers earplugs, and our travel article covers hotel rooms.

A dark-bedroom audit you can do tonight

We found no trial testing blackout curtains for health. Researchers recommend them because they block the kind of light the studies measured¹.

Try this. Turn off every light, get into bed, and wait five minutes for your eyes to adjust. Then look around.

Light source	How to spot it	Fix	Cost
Streetlights or security lights	A glow on the ceiling, walls or your face	Blackout curtains or shades; move the bed so outside light misses your face1	Low to higher
Light under the door	A bright line at the floor	Close the door; roll a towel along the gap	Free
TV left on to fall asleep	Ask yourself or your partner	Use a sleep timer or move the TV out3	Free
Bedside or overhead lamp	Any lamp still on at sleep time	Switch it off; if you need light, use dim, low amber or red1	Free
Glowing clocks and chargers	Small dots of light	Turn the clock face out of view13; tape over tiny standby lights, but never a smoke alarm	Free
Early summer sunrise	A bright room before your alarm	Blackout shades or an eye mask18	Low

A mask is the cheapest fix of all, at $10 to $20⁷. If you use blackout shades, open them soon after you wake. Our article on morning light explains why daylight matters. Night-shift workers can find a daytime sleep plan in our article on irregular schedules.

Older adults need darkness and a safe path

This is the tension. Darkness helps sleep, but an older adult who gets up at night needs to see the floor. Writing in the journal Gerontology in 2020, researchers noted that poor lighting at night raises the risk of falls¹⁰.

They tested motion-activated LED strips placed along the route from bed to bathroom. The 64 participants were 65 or older, with an average age of about 81, and used the lights for about four months on average¹⁰.

Nighttime fear of falling dropped from 5.5 to 3.8 on a 10-point scale, and sleep-quality ratings rose from 6.7 to 7.4¹⁰. Falls were too rare to judge, and the study had no comparison group, so treat the results as early¹⁰.

The Lighting Research Center at Rensselaer Polytechnic Institute tried a similar idea in four suites of a long-term care home in Clifton Park, New York, according to the trade publication McKnight’s¹⁵.

Strips of amber light ran along the floor beneath the bed frame, outlined the bathroom doorway and lit the sink and toilet grab bar¹⁵. When someone got up, motion sensors faded the strips on gently¹⁵. Floor light measured roughly 10 to 15 lux beside the bed and 2 to 10 lux at the bathroom door¹⁵. Every resident surveyed said the amber glow didn’t wake them¹⁵.

A safe-path night-lighting plan

1. Walk the route at night from bed to toilet with the lights off, and note every turn, step and rug edge.
2. Place motion-activated lights low, near the floor, along that route¹¹⁰.
3. Choose amber or red, not white or blue¹.
4. Keep them dim: enough to see the floor, not enough to read by¹¹⁵.
5. Point them at the floor, away from the sleeper’s eyes¹.
6. Keep a flashlight or switch within reach of the bed for emergencies.
7. Clear the path of cords, loose rugs and clutter.

Children’s nightlights: the nearsightedness scare that didn’t hold up

In May 1999, researchers from the University of Pennsylvania and Children’s Hospital of Philadelphia reported in Nature that children who slept with lights on before age 2 were more likely to become nearsighted¹¹¹². The strongest link was with full room lighting¹². Nearly half the children who slept in fully lit rooms became nearsighted¹¹.

Less than a year later, two independent teams, funded by the National Eye Institute, reported in Nature that they found no link¹². Karla Zadnik’s Ohio State team studied 1,220 children in community settings¹¹. About 20% of those who slept in the dark became nearsighted, compared with 17% who used a nightlight and 22% who slept in a fully lit room, a group of only 45 children¹¹.

What explained the first result? Nearsighted parents were more likely to leave a light on, and nearsightedness runs in families¹¹¹². The original study hadn’t accounted for parents’ eyesight, and its children came from a specialty clinic¹¹.

It’s a textbook case of confounding. It’s also a reason to hold the adult light-at-night studies to the same standard until trials test them.

If a nightlight helps your child feel safe, use a dim, warm-colored one placed low and away from the bed, in line with the Northwestern team’s general advice¹. Our article for busy parents covers bedtime anchors.

What the evidence supports

Claim	Evidence level	What the research shows
One night of room light during sleep raises heart rate and next-morning insulin resistance	Promising but early	One small randomized lab study of 20 volunteers12
Sleeping with a light or TV on leads to weight gain	Promising but early	Large observational study of women; confounding still possible3
Bright nights raise diabetes and heart disease risk	Promising but early	Large wrist-sensor studies with graded links; observational56
Light during sleep in older adults is tied to obesity, diabetes and high blood pressure	Promising but early	One snapshot; illness may lead to light use4
An eye mask improves next-day memory and alertness	Promising but early	Small effects in healthy young adults89
Blackout curtains improve health	Not enough evidence	Recommended by researchers; not tested directly1
A small, dim nightlight harms adults	Not enough evidence	No link to weight gain in the Sister Study3
Motion-sensor path lights prevent falls	Not enough evidence	Less fear of falling and better sleep ratings; falls too rare to measure10
Nightlights cause nearsightedness in children	Evidence says no, or unsafe	Two follow-up studies found no link; parents’ eyesight explained the first finding1112

Darkness is free, but some sleep problems need more

Natural health often means restoring something modern life took away. Night darkness may be the easiest to win back. It needs no supplement and no subscription, and the main tools are a light switch, a curtain and a mask.

The science is young, and most of it is observational. But darkness carries little risk, as long as you keep a safe path lit.

If you keep a light on because the dark makes you anxious, that deserves care, not shame. Our upcoming article on nighttime anxiety covers it.

Talk to a clinician if you notice any of these:

• loud, frequent snoring with pauses in breathing, gasping or choking¹³;
• trouble sleeping at least three nights a week for three months or more¹⁴;
• regularly taking more than 30 minutes to fall asleep, or feeling sleepy during the day¹³;
• prickling or tingling in your legs in the evening or at bedtime that eases when you move¹³;
• acting out your dreams¹³.

If you feel drowsy behind the wheel, pull over¹³. Our upcoming articles on sleep disorders cover each of these.

Questions readers ask

The bottom line

Darkness is a signal, and your body reads it even while you sleep. One small experiment showed that a single night of room light can raise heart rate and insulin resistance by morning. Large studies tie bright nights to weight gain, diabetes and heart disease, though they can’t prove the light is the cause.

You don’t have to wait for proof to act, because the fixes are cheap and safe. Turn off the TV and lamps, block outside light with shades or a mask, and keep any light you need dim, low and amber. For older adults, light the path to the bathroom, not the bedroom. And rest easy about your child’s nightlight: the nearsightedness scare didn’t hold up.

Related reading

• Your body sheds heat to fall asleep, and the 60 to 67°F rule is only a rough guide
• One in seven workers clocks in during off-hours, and sleep is the first thing to anchor
• One side of your brain keeps watch on your first night away
• Why observational studies got smoking right and hormone therapy wrong
• Busy parents need a few anchors more than a perfect routine

Sources

1. Northwestern University. “Close the Blinds During Sleep to Protect Your Health” (on Mason IC, Grimaldi D, Reid KJ, et al., “Light Exposure During Sleep Impairs Cardiometabolic Function,” PNAS, 2022;119(12)). Northwestern Now, March 14, 2022. news.northwestern.edu
2. Koç University. “Do You Sleep With the Lights On?” Kurious, Koç University (summary of Mason et al., PNAS, 2022). kurious.ku.edu.tr
3. National Institutes of Health. “Artificial Light During Sleep Linked to Obesity” (on Park YM, White AJ, Jackson CL, Weinberg CR, Sandler DP, JAMA Internal Medicine, 2019). NIH Research Matters, June 18, 2019. nih.gov
4. Northwestern University. “Light During Sleep in Older Adults Linked to Obesity, Diabetes, High Blood Pressure” (on Kim M, Vu T-H, Maas M, et al., SLEEP, 2022). Northwestern Now, June 27, 2022. news.northwestern.edu
5. Windred DP, Burns AC, Rutter MK, et al. “Personal Light Exposure Patterns and Incidence of Type 2 Diabetes: Analysis of 13 Million Hours of Light Sensor Data and 670,000 Person-Years of Prospective Observation.” The Lancet Regional Health – Europe, 2024; DOI 10.1016/j.lanepe.2024.100943 (abstract and article excerpts reproduced, with a Medscape news report, by Dagens Diabetes, July 14, 2024). dagensdiabetes.se
6. Windred DP, Burns AC, Rutter MK, Lane JM, Saxena R, Scheer FAJL, Cain SW, Phillips AJK. “Light Exposure at Night and Cardiovascular Disease Incidence.” JAMA Network Open, 2025;8(10):e2539031 (Flinders University research record). researchnow.flinders.edu.au
7. Zhou E. “Does Sleeping With an Eye Mask Improve Learning and Alertness?” Harvard Health Publishing, February 21, 2024. health.harvard.edu
8. Greco V, Bergamo D, Cuoccio P, Konkoly KR, Lombardo KM, Lewis PA. “Wearing an Eye Mask During Overnight Sleep Improves Episodic Learning and Alertness.” SLEEP, 2023;46(3):zsac305 (Cardiff University ORCA record). pubmed.ncbi.nlm.nih.gov
9. Greco V, et al. “Wearing an Eye Mask During Overnight Sleep Improves Episodic Learning and Alertness.” bioRxiv (preprint version), 2022. biorxiv.org
10. Thölking TW, Lamers ECT, Olde Rikkert MGM. “A Guiding Nightlight Decreases Fear of Falling and Increases Sleep Quality of Community-Dwelling Older People: A Quantitative and Qualitative Evaluation.” Gerontology, 2020;66(3):295–303. karger.com
11. Ohio State University. “Night Lights Don’t Lead to Nearsightedness, Study Suggests.” Ohio State News, March 2, 2000. news.osu.edu
12. National Eye Institute. “Myopia Development and Nighttime Light Exposure in Infancy.” NEI, March 1, 2000. nei.nih.gov
13. National Heart, Lung, and Blood Institute. “Your Guide to Healthy Sleep.” NIH Publication No. 11-5271, revised August 2011. nhlbi.nih.gov
14. National Heart, Lung, and Blood Institute. “Insomnia: Diagnosis.” NIH, updated March 24, 2022. nhlbi.nih.gov
15. Chotiner M. Article on the Lighting Research Center’s night-lighting system for long-term care. McKnight’s Long-Term Care News, undated. mcknights.com