Blackout curtains and sleep masks both reduce the light that reaches your eyes in the bedroom. Product pages describe them with labels such as "99.99% blackout" or "total blackout". Those are the maker's own figures, and each maker measures under its own conditions. Whether they improve your sleep depends on you and your room, and no result is guaranteed.
| What to check | What to look for | What gets missed |
|---|---|---|
| Label | The percentage and the test method, beyond a word like "blackout" | The figure describes the fabric and leaves out the gaps around the window |
| Color | Darker colors let less light through | A pale curtain can look brighter even at the same stated figure |
| Size | Width and length from the window frame, beyond the rod | Sizing to the rod alone leaves gaps at the sides, top and bottom |
| Fullness | Fabric about 1.5 times the finished width | A flat panel pulls open at the center seam |
| Weight and mounting | The load rating of the rod or tension rod | Thick panels are heavy, and a tension rod can drop them |
| Returns | Whether opened curtains can be returned or exchanged for another size | A measuring mistake that cannot be undone |
Reading blackout labels and the conditions behind them
Japan grades curtains by class for light blocking, with Class 1 the darkest. The scale is set by the Japan Interior Fabrics Association (NIF). A 2014 note from the Gifu Prefectural Research Institute of Manufacturing Information Technology lists Class 1 as 99.99% blocking or higher, Class 2 as 99.80% up to 99.99%, and Class 3 as 99.40% up to 99.80%. The figure comes from JIS L 1055, the Japanese standard test for curtain light blocking. The test compares the illuminance inside a box with and without a fabric sample over the opening, normally under 10,000 lux and, for very dark fabrics, under 100,000 lux.
A grade printed on a sales page can differ from what a test finds. In February 2024 the National Consumer Affairs Center of Japan tested a curtain sold online as "Class 1". Measured with the JIS L 1055 method and compared against the NIF criteria, some areas of the fabric came out at Class 3 and others fell below it. The curtain did meet the "90% or higher" figure printed on the page, so the number held while the grade differed from the scale most Japanese makers use. The agency advises buyers to check which standard a grade was judged against, because some online listings use a different one.
I verified the Japanese scale and no other, so for any label, read what it actually says:
- With a bare word such as "blackout" or "Class 1", find the percentage and the standard or test method behind it.
- A figure such as "99.99%" ranks two products when both state the same test method.
- "Total blackout" and "100%" are each maker's own wording for a level. Look for the test method on the page or ask the seller.
Even the strictest Japanese grade allows up to 0.01% of light through. A Japanese listing for a Class 1 curtain says as much: first-grade blackout does not stop light completely, and a dark color is the better choice if light leaks bother you. If you want a room close to dark, the gaps around the window matter more than the grade of the fabric.
Measuring and mounting to reduce light leaks
The label describes the fabric. Light that comes around the curtain depends on the gaps instead, and it enters above the rod, at the left and right edges, where two panels meet, and below the hem.
How the window shape changes where light gets in
A sliding glass door or a floor-length window needs a curtain that reaches nearly to the floor. Light from a hallway or garden comes in under a short curtain, so a longer panel reduces it.
A standard window leaves wall below the frame, so a panel that hangs well below the sill blocks light from underneath. If the rod sits just above the frame, the top stays open, so check whether you can mount it higher than the frame.
A small window often gets a ready-made curtain far longer than needed. Some makers sell shorter sizes for small windows. For a window that opens outward, check that the length does not get in the way of opening it.
Measure from the window frame
Start from the frame, not from the length of the rod. If the rod is narrower than the frame and you buy to the rod width, light comes in at both edges. Adding several centimeters on each side of the frame width reduces edge leaks. That margin is a figure commonly seen in retailers' measuring guides, and I did not find an official standard behind it.
Fullness also affects the gap when the curtain is closed. A Japanese product page for a Class 1 curtain says that using fabric 1.5 times the finished width raises airtightness and improves light blocking and insulation. A flat, barely gathered curtain tends to open at the center.
Choosing how to mount it
If you own the home and can drill into the wall or frame, mount the rod higher than the frame to close the top gap. If you rent, ask your landlord or property manager whether drilling is allowed. Where it is not, tension rods and adhesive options exist, but blackout fabric is thick and heavy. One pair of Japanese Class 1 curtains lists a weight of about 1.7 kg for the two panels, so check the rod's load rating and be careful with a tension rod on a large window.
Check return terms and whether you can remeasure
Wrong sizing is the costly mistake with blackout curtains. Before ordering, check whether you can return them after opening, exchange them for another size, and who pays the return shipping. Some curtains cannot be returned after washing or ironing. Measure the frame width and the height on two different days, and order once both measurements agree.
Heat and insulation, morning light and alternatives
Insulation is judged differently in summer and winter
Many blackout curtain pages mention insulation, warmth, energy saving or lower bills. One Japanese listing says its curtain blocks heat in summer and keeps heat in during winter, and the page gives no window or conditions for the claim.
In summer a curtain can block sunlight and limit the rise in room temperature, and in winter it can slow the cold from outside. The result depends on the glass, how the curtain hangs, which way the window faces and how well the home is insulated, so this article does not claim that blackout curtains reduce heating or cooling costs. I could not find a public test that gives the insulation effect of a blackout curtain alone as a number. If insulation is your goal, choose a product that states its specification and test conditions.
Blocking too much light removes morning light
A dark bedroom also removes the light you may want when you wake up. Japan's Ministry of Health, Labour and Welfare publishes a guide, e-Healthnet, which says that light in the morning moves the body clock earlier, that people who find it hard to fall asleep or to get up should aim for plenty of light after waking and through the morning, and that the first thing to do after getting up is to open the curtains and let natural light in.
A curtain that stays shut after you wake reduces that light. Opening it at your wake time, or planning to get morning light somewhere else, keeps both a dark bedroom and morning light. In summer, when sunrise is early, the room gets bright before your alarm, so that is the season when blackout is easiest to notice.
What can stand in when you have no curtains
If you rent and cannot put up a rod, or you travel often, a sleep mask can take the curtain's place, and earplugs cover noise.
A trial by Hu and colleagues, published in 2010, tested this pair. Fourteen healthy volunteers slept in a simulated intensive care unit environment with recorded noise and light. On the nights they used earplugs and eye masks, they had more REM sleep and fewer arousals, and their urinary melatonin was higher. The subjects were healthy adults in a simulated hospital setting, so the result does not carry over directly to your own bedroom.
For a sleep mask, fit and side pressure matter more than the stated light blocking. A shape that does not fit your face lets light in at the nose and cheeks, and one that presses hard on the eyelids is uncomfortable. A contoured mask that clears the eyelashes, with a strap that does not dig in behind the ears, is easier to keep on until morning. That is a comfort point from experience with product descriptions, and the research above does not address it.
How to check whether it helps your sleep
Studies connect bedroom light to health, but I did not find one showing that switching to blackout curtains makes people sleep better.
A study of older adults in Nara, Japan (Obayashi and colleagues, 2018) measured bedroom light with sensors and followed 863 people with an average age of 71.5. Those with average light of 5 lux or more during sleep developed depressive symptoms at a higher rate than those below 5 lux, with a hazard ratio of 1.89 (95% confidence interval 1.13 to 3.14). It is an observational study, so light cannot be named as the cause. What it shows is an association between brighter bedrooms and later depressive symptoms.
To test it in your own bedroom, change one thing and keep a record.
- For a week before the change, write down in the same way how long it takes to fall asleep, how often you wake at night and how you feel in the morning.
- At night, switch the lights off and let your eyes adjust for about five minutes, then see where around the window light shows. Look at the top, the sides, the center seam and the bottom.
- Change either the curtain or the mask, and keep the same record for the next one to two weeks.
- Compare the two records. If the difference is small, light may not be the main cause of your sleep problem.
If you still cannot fall asleep after two weeks, or you feel strongly sleepy during the day, the brightness of your bedroom may not be the cause. Talk to a clinic that deals with sleep. This article is general guidance and not medical advice.
Frequently asked questions
Will a Class 1 curtain make the room completely dark?
No. Class 1 means 99.99% blocking or higher, and light can still enter around the window, above the rod and where two panels meet. To get closer to dark, look at measuring and mounting as well as the grade.
Is a curtain labeled 100% darker than one labeled 99.99%?
There is no basis for saying so. The figure is the maker's own, and conditions differ between makers. A product page rarely shows whether "100%" was measured under a stricter standard than the top grade, so look for the test method.
Do blackout curtains lower energy bills?
That cannot be stated. The result depends on the window, how the curtain hangs, the direction the window faces and the home's insulation. Many product pages describe insulation without giving test conditions.
Will blackout curtains make it harder to wake up?
Some people feel less alert on waking because less morning light reaches them. Opening the curtain at your wake time, or pairing it with a sheer curtain, lets morning light in. Japan's e-Healthnet also recommends opening the curtains first thing after getting up.
Are a sleep mask and earplugs enough?
A trial in a simulated intensive care unit found better sleep with earplugs and eye masks. It had 14 healthy adults and a setting unlike a home bedroom. Try it yourself and record whether falling asleep and night waking change.
Sources
- National Consumer Affairs Center of Japan, "Curtain whose light-blocking labeling differed from the standard widely used in Japan" (Tests for Resolving Consumer Complaints, No. 184), 16 February 2024. https://www.kokusen.go.jp/news/data/n-20240216_2.html (Japanese)
- Gifu Prefectural Research Institute of Manufacturing Information Technology, "Sangi-se news, October 2014", introduction of the light-blocking test machine (NIF grades and the JIS L 1055 method A). https://www.gitec.rd.pref.gifu.lg.jp/files/letters/2014/letter_iri_2014_2.pdf (Japanese)
- Japan's Ministry of Health, Labour and Welfare, e-Healthnet, "Good sleep and lifestyle habits". https://kennet.mhlw.go.jp/information/information/heart/k-01-004.html (Japanese)
- Obayashi K, Saeki K, Kurumatani N. Bedroom Light Exposure at Night and the Incidence of Depressive Symptoms: A Longitudinal Study of the HEIJO-KYO Cohort. American Journal of Epidemiology 187(3), 2018, 427-434. https://pubmed.ncbi.nlm.nih.gov/28992236/
- Hu RF, Jiang XY, Zeng YM, Chen XY, Zhang YH. Effects of earplugs and eye masks on nocturnal sleep, melatonin and cortisol in a simulated intensive care unit environment. Critical Care 14(2), 2010, R66. https://pubmed.ncbi.nlm.nih.gov/20398302/