Blackout Curtains for Better Sleep: Do They Actually Help?

Yes. Blackout curtains for better sleep are worth it if streetlights, early dawn or a neighbor’s floodlight reach your bed. No trial has tested curtains directly, but in lab studies, sleeping with the light on made sleep lighter and raised next-morning insulin resistance. The goal is a bedroom under 1 lux at your eyes.

What does light at night do to your sleep?

Your eyelids are thin. They pass only an estimated 5 to 9% of light,¹ yet that is enough to matter. In a small lab study, green light shone through closed eyelids from a light mask cut melatonin by 36 to 56% compared with a dark night, depending on the time of night.²

Sleep itself gets shallower too. In a sleep lab, 10 healthy adults slept one night with the lights off and one with a bedside light on. With the light on, they had more stage 1 sleep, less slow-wave sleep and more arousals.³

The circadian system is also more sensitive than most people assume. In a study of 55 young adults exposed to light in the evening (eyes open), half of the maximum melatonin suppression happened at only 24.6 lux, well within typical indoor lighting. The most sensitive person reached that point at 6 lux.⁴ That study was done awake, not asleep, but it explains why researchers treat even “dim” bedroom light as a variable worth controlling.

Does sleeping with light on affect blood sugar?

The clearest lab evidence comes from a 2022 study at Northwestern University. Twenty healthy adults were randomized to two nights in a sleep lab. One group slept both nights in dim light (under 3 lux). The other slept the first night in dim light and the second night with overhead room lights on at 100 lux.¹

After the lit night:

• Insulin resistance rose. Morning HOMA-IR went up about 15% in the room-light group and fell about 4% in the dim group (P = 0.018).¹
• Heart rate went up during sleep, and heart rate variability shifted toward the “fight or flight” side of the nervous system.¹
• Sleep got lighter. The room-light group spent more time in stage N2 and less in REM, and their slow-wave sleep rose less than in the dim group.¹

One detail often left out of the headlines: melatonin did not change measurably. The authors suggest the metabolic effect may run through the nervous system rather than melatonin.¹ It was also one night, in 20 young, healthy people. It shows a mechanism, not a long-term health outcome.

What do large studies of light at night show?

Long-term data come from observational studies. They cannot prove that light causes the problems, since people with lit bedrooms may differ in other ways, but the pattern is consistent.

Study	People	Light measure	Main finding
Windred 2024, UK Biobank5	84,790 adults	Wrist light sensor, 1 week	Brightest 10% of nights: 53% higher type 2 diabetes risk (HR 1.53) vs. darkest half
Park 2019, Sister Study6	43,722 women	Self-report	Light or TV on in the room: 17% higher risk of gaining 5 kg or more over about 5.7 years
Kim 20237	552 adults aged 63 to 84	Wrist sensor, 7 days	Any light in the darkest 5 hours: higher odds of obesity (OR 1.82), diabetes (OR 2.00), hypertension (OR 1.74)
Obayashi 20138	528 older adults	Bedroom meter, 2 nights	Average of 3 lux or more: higher odds of obesity (OR 1.89) and dyslipidemia (OR 1.72)

In the UK Biobank study, the gap in diabetes risk between bright and dark nights was about as large as the gap between people with low and moderate genetic risk.⁵

Does outdoor light pollution affect sleep?

Streetlights are exactly what blackout curtains are for, and population studies tie them to sleep timing. In a US telephone survey of 19,136 adults matched to satellite light maps, people in brighter areas went to bed and woke later, slept less, felt sleepier in the day and were less satisfied with their sleep.⁹

Teenagers show the same pattern. In a nationally representative sample of 10,123 US adolescents, those in the brightest quarter of outdoor light at night went to bed 29 minutes later on weeknights than those in the darkest, and slept 11 minutes less.¹⁰

These studies measure light outside the home from space, not inside the bedroom. Curtains break that link, which is the point.

How dark should your bedroom be?

In 2022, an international group of 18 circadian scientists published lighting recommendations. For the sleep environment, the maximum is 1 lux melanopic EDI, measured at the eye. They name blackout blinds as one way to get there. If you need light to move around at night, they set the ceiling at 10 lux.¹¹

Melanopic EDI weights light by how strongly it drives the body clock. For warm white light it sits well below the plain lux reading; for daylight the two are about equal. Some cool or color-mixed LEDs read higher, so aim for a meter reading under 1 lux, and lower still if the light leaking in is cool white.¹¹

How to check your room:

1. Use a meter at pillow height. Point it where your face rests, lights off, at the darkest hour you sleep. A meter that reads below 1 lux is needed; see our light meter picks and the lux levels reference.
2. Do the eye test. Lie in bed for 10 minutes with the lights off. If you can read a clock face or see the edges of the curtain glow, light is getting in.
3. Check at dawn. In summer, light can start leaking in hours before your alarm. Look at the room around sunrise once.

For the whole day-to-night picture, see how many lux you need.

Why do blackout curtains still let light in?

The fabric is rarely the problem. A true blackout curtain blocks nearly all light through the cloth. Light gets in around it: at the top above the rod, along the sides, through the gap in the middle and under the hem. That glowing outline is where most fixes should focus.

Room-darkening curtains are a different product. They cut glare but let a visible amount of light through. If the listing says “room darkening” rather than “blackout,” expect a lit room on bright nights. Our blackout curtain picks list the specs to check.

How do you block light gaps around curtains?

Fix	Blocks	Cost	Notes
Wider, longer curtains	Side and bottom glow	Low	Overlap the frame on each side and reach the sill or floor
Wraparound (return) rod	Side gaps	Low	The curtain curves back to the wall, closing the edge
Mount the rod high or add a valance	Top gap	Low	A cornice or valance boards over the light above the rod
Magnetic or hook-and-loop tape	Side and center gaps	Low	Sticks the curtain edges to the wall or to each other
Blackout liner	Light through thin curtains	Low	Hangs behind curtains you already own
Inside-mount blackout shade plus curtain	Most gaps	Medium	Two layers catch what one misses
Blackout window film	Light through glass	Low to medium	Good for skylights, door panes and odd shapes
Tape over standby LEDs	Device light	Very low	Chargers and TVs often glow brighter than you’d expect in a dark room
Door draft stopper	Hallway light	Very low	Blocks the strip under the door

A few practical points. Static-cling window film peels off cleanly, which suits renters. Adhesive film is harder to remove, so check the maker’s glass compatibility notes before applying it to double-pane windows. Don’t board up a bedroom window permanently: it is an emergency exit.

If young children live in the home, pick cordless blinds and shades. The US Consumer Product Safety Commission counted 46 child strangulation deaths from window covering cords between 2014 and 2019, and says cordless coverings, or ones with inaccessible cords, are the only safe option.¹²

Is a sleep mask as good as blackout curtains?

For blocking light, a well-fitted mask is often better, because it also covers the phone screen, the standby LED and the hallway light. It is also the cheapest option and the only one that travels.

The evidence is modest but real. In a crossover study of 94 adults aged 18 to 35, a week of wearing an eye mask during sleep, compared with no mask or a mask with eye holes cut out, improved next-day word-pair learning and reaction time. The effects were small (Cohen’s d 0.16 to 0.19). Sleep duration did not change.¹³ In intensive care patients, a systematic review of 19 studies (1,379 people) found earplugs and eye masks improved self-reported sleep quality, though the objective measures were not consistent.¹⁴

Masks have downsides. Some people can’t stand them, they slip off, and they press on the eyes. Curtains darken the whole room for a partner and need nothing on your face. Our sleep mask picks cover contoured designs that keep pressure off the eyelids.

Who should use blackout curtains for better sleep?

• Shift workers sleeping in the day. Daylight is far brighter than streetlight, so day sleepers need the most blocking. See our guide to shift work sleep.
• City dwellers with streetlights, signs or security lights outside the window.
• Summer early risers by accident. At high latitudes, summer dawn can arrive hours before your alarm.
• Light sleepers who wake at the first sign of light.

If your bedroom is already dark, curtains add little. Measure first.

Do blackout curtains have any downsides?

The main cost is morning light. Bright light after waking helps anchor your body clock, and a sealed room can make it harder to get up. Open the curtains as soon as you wake, or step outside. Our guide to morning sunlight explains why this matters.

The other risk is shifting the problem indoors. Blacking out the window does little if you spend the last hours of the evening under bright overhead light. Room light before bed delayed melatonin onset in 99% of people in a lab analysis of 116 adults and shortened melatonin duration by about 90 minutes.¹⁵ Dim and warm the lights in the evening too; see the best light color for sleep.

This guide is general information, not medical advice. If you have ongoing insomnia, loud snoring or daytime sleepiness despite a dark room, talk to a doctor.

Footnotes

1. Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. PNAS. 2022;119(12):e2113290119. doi:10.1073/pnas.2113290119 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. Figueiro MG, Rea MS. Preliminary evidence that light through the eyelids can suppress melatonin and phase shift dim light melatonin onset. BMC Research Notes. 2012;5:221. doi:10.1186/1756-0500-5-221 ↩

3. Cho JR, Joo EY, Koo DL, Hong SB. Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms. Sleep Medicine. 2013;14(12):1422-1425. doi:10.1016/j.sleep.2013.09.007 ↩

4. Phillips AJK, Vidafar P, Burns AC, et al. High sensitivity and interindividual variability in the response of the human circadian system to evening light. PNAS. 2019;116(24):12019-12024. doi:10.1073/pnas.1901824116 ↩

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. Lancet Regional Health Europe. 2024;42:100943. doi:10.1016/j.lanepe.2024.100943 ↩ ↩²

6. Park YM, White AJ, Jackson CL, Weinberg CR, Sandler DP. Association of exposure to artificial light at night while sleeping with risk of obesity in women. JAMA Internal Medicine. 2019;179(8):1061-1071. doi:10.1001/jamainternmed.2019.0571 ↩

7. Kim M, Vu TH, Maas MB, et al. Light at night in older age is associated with obesity, diabetes, and hypertension. Sleep. 2023;46(3):zsac130. doi:10.1093/sleep/zsac130 ↩

8. Obayashi K, Saeki K, Iwamoto J, et al. Exposure to light at night, nocturnal urinary melatonin excretion, and obesity/dyslipidemia in the elderly: a cross-sectional analysis of the HEIJO-KYO study. Journal of Clinical Endocrinology & Metabolism. 2013;98(1):337-344. doi:10.1210/jc.2012-2874 ↩

9. Ohayon MM, Milesi C. Artificial outdoor nighttime lights associate with altered sleep behavior in the American general population. Sleep. 2016;39(6):1311-1320. doi:10.5665/sleep.5860 ↩

10. Paksarian D, Rudolph KE, Stapp EK, et al. Association of outdoor artificial light at night with mental disorders and sleep patterns among US adolescents. JAMA Psychiatry. 2020;77(12):1266-1275. doi:10.1001/jamapsychiatry.2020.1935 ↩

11. Brown TM, Brainard GC, Cajochen C, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biology. 2022;20(3):e3001571. doi:10.1371/journal.pbio.3001571 ↩ ↩²

12. US Consumer Product Safety Commission. Go cordless with window coverings to avoid risk of strangulation to infants and children. 2020. cpsc.gov ↩

13. Greco V, Bergamo D, Cuoccio P, et al. Wearing an eye mask during overnight sleep improves episodic learning and alertness. Sleep. 2023;46(3):zsac305. doi:10.1093/sleep/zsac305 ↩

14. Locihová H, Axmann K, Padyšáková H, Fejfar J. Effect of the use of earplugs and eye mask on the quality of sleep in intensive care patients: a systematic review. Journal of Sleep Research. 2018;27(3):e12607. doi:10.1111/jsr.12607 ↩

15. Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology & Metabolism. 2011;96(3):E463-E472. doi:10.1210/jc.2010-2098 ↩