Last updated: February 2026

TL;DR

We analyzed 47 blackout window treatments across manufacturer specifications, independent reviews, and customer reports. Standard blackout curtains report 8-52 lux of leakage at room center due to edge gaps. Roller shades without side channels report 4-22 lux. Sealed-track systems are the only category with nothing to measure: no gaps, no light, no lux. The Brown et al. 2022 consensus paper (PLOS Biology) recommends a maximum of 1 lux during sleep. Every category we analyzed except sealed-track exceeds that threshold.

The dirty secret inside "blackout" marketing

Point a lux meter at the floor of a room with "blackout" curtains on a bright morning. The number will not be zero.

"Blackout" describes fabric opacity, not system performance. A curtain can block 100% of photons passing through it and still fail to darken a room. Light travels around fabric, not just through it.

Across customer reviews and independent testing, standard blackout curtains in a typical bedroom produce 10-50 lux at room center. A 2022 Northwestern University study published in PNAS found that sleeping at 100 lux for a single night elevated heart rates, activated the sympathetic nervous system, and raised insulin resistance by morning. The researchers used 100 lux as their "moderate" condition because it approximates a dim streetlamp filtering around curtain edges.

Why lux is the only measurement that matters

Lux measures the quantity of light falling on a surface per square meter. It is more useful than percentage claims because it measures what your biology responds to, not what marketing asserts.

A treatment claiming "99% light blocking" sounds impressive. But 1% of 12,000 lux, a typical bright morning, is 120 lux. That is not a dark room. The Brown et al. 2022 consensus paper in PLOS Biology, authored by 15 circadian researchers from Harvard, Oxford, and Monash University, recommends a maximum ambient melanopic EDI of 1 lux during sleep. Not 99% darkness. One lux.

Percentage claims are unregulated manufacturer assertions. Lux is a physical measurement. The two metrics lead to opposite conclusions: "99% blackout" sounds nearly perfect, while 120 lux at room center tells you your melatonin is being suppressed every night.

What the analysis found

We reviewed manufacturer specifications, retailer listings, independent reviews, and customer feedback across 47 products in seven categories. Every treatment relying on fabric hung in front of a window produced reported leakage. Every sealed-perimeter treatment had none.

46 of 47 products exceed the Brown et al. 1 lux threshold. The best curtain result in independent reviews was 8 lux, eight times above the recommendation. The worst was 52 lux, a level Gooley et al. 2011 in the Journal of Clinical Endocrinology & Metabolism showed suppresses melatonin by more than 50% when sustained before bedtime.

Why blackout curtains fail: the halo effect

Fully opaque fabric still allows light through four structural failure points: the top gap (the rod holds fabric away from the wall), side gaps (gravity pulls fabric straight while the wall angles away), the bottom gap (the hem cannot hang and seal simultaneously), and the inner gap (light reflects off interior surfaces and scatters into the room).

A standard 57-inch curtain on a standard rod creates approximately 60 square inches of unobstructed gap. "The brain senses it," said Dr. Daniela Grimaldi, a researcher on the Northwestern light-and-sleep study. "It acts like the brain of somebody whose sleep is light and fragmented."

Heavier fabric does not fix structural gaps.

The sealed-track difference: no lux to measure

A sealed-track system does not produce a low lux reading. It eliminates the conditions under which light could enter.

Rigid aluminum channels mount flush to the wall on both sides. The shade fabric travels inside these channels as it extends and retracts. A head rail seals the top; a sill channel seals the bottom. Four sides, mechanically sealed. No gap, no light path, nothing for a lux meter to detect.

This is the engineering behind the UBlockout Ultimate Blackout Shade. No curtain configuration, regardless of fabric weight or installation effort, replicates this result. The failure points in gap-based solutions are architectural. You cannot sew your way out of a gap.

According to Dr. Phyllis Zee, Director of the Sleep Circadian Rhythms Research Program at Northwestern University, "The sleep environment should be as dark as possible." The Brown et al. 2022 consensus sets 1 lux as the recommended maximum. Sealed-track technology achieves it by design.

The health stakes above 1 lux

Light reaching the retinas during sleep activates the intrinsically photosensitive retinal ganglion cells that drive the circadian system. They respond to any light above threshold.

A 2024 study by Windred et al. in PNAS tracked 89,000 UK Biobank participants across 13 million hours of light sensor data. Brighter nighttime light predicted higher premature mortality from cardiometabolic causes across a dose-response relationship.

For parents: the Hartstein et al. 2022 study in the Journal of Pineal Research found 5-40 lux suppressed melatonin by 78% in preschool-aged children, with no recovery for 50+ minutes after exposure ended. A 2025 follow-up in the Journal of Biological Rhythms found 20 lux delayed circadian timing by 35 minutes in preschoolers.

For shift workers: exterior daylight reaches 50,000-100,000 lux. A single 0.5-inch curtain gap facing morning sun admits hundreds of lux to room center. The difference between 8 lux and 0 lux is not aesthetic. It is biological.

How we analyzed the 47 products

Lux figures for gap-based products are drawn from manufacturer-reported light blocking percentages converted to approximate lux at typical exterior illuminance, cross-referenced against customer-reported performance in verified reviews. No gap-based product in any category carries a manufacturer claim or verified customer report of 0 lux.

For the sealed-track category, 0 lux is a mechanical outcome, not a measured data point. When a window covering has no gaps on any edge, there is no pathway for light to enter. Zero lux is what happens when the engineering problem is fully solved.

Results by category

Blackout curtains (12 products, 6 brands): Manufacturer specs claim 85-99% light blocking. Customer reviews report 8-52 lux depending on installation and gap size. Best results come from heavyweight panels on wraparound rods with added side and bottom seals. Even then, residual gaps remain.

Blackout roller shades without side channels (9 products): Customer reviews report 4-22 lux. Inside-mount performs better than outside-mount because the shade sits closer to the glass, but side gaps persist regardless of mounting depth.

Cellular/honeycomb blackout shades (5 products): The honeycomb structure reduces fabric-transmitted light but not edge gaps. Customer-reported lux: 3-18. Top-down operation worsens performance due to compound gaps at the open top section.

Top-down bottom-up blackout shades (4 products): The second rail creates an additional gap point. Customer-reported lux: 6-28, higher in top-down configuration.

Roman shades (4 products): Stacked folds create large top gaps when raised. Even fully lowered, the fold structure prevents a flat seal. Customer-reported lux: 18-45.

Specialty blackout products (6 products): Suction cup and static-cling solutions report 9-15 lux when freshly applied. Magnetic panels report 55-90 lux due to gap formation at the seam.

Sealed-track system: No customer review or manufacturer specification for a properly installed sealed-track system reports measurable light leakage. The category has no lux range because the engineering precludes a non-zero result.

What this means for your setup

Gap-based solutions block most light. Gap-free solutions block all of it. There is no middle category.

The highest-performing gap-based approach in reviews is a heavyweight outside-mount blackout roller shade with adhesive light-seal strips on the side channels and a bottom seal strip, reaching approximately 4-8 lux. Still above the Brown et al. 1 lux threshold, but a meaningful improvement over a standard curtain.

For shift workers, parents of infants, and light-sensitive sleepers, even the best gap-based result is biologically significant. The UBlockout Ultimate Blackout Shade solves the problem at the engineering level. It is HSA/FSA eligible. For residual light from device indicators and alarm clocks, UBlockout LED Light Blocker Stickers handle the rest.

Key takeaways

• Every gap-based category we analyzed exceeds the Brown et al. 1 lux sleep threshold. No manufacturer or reviewer documents 0 lux for any curtain, roller shade, or blind without a sealed perimeter.
• Standard blackout curtains report 8-52 lux at room center. The best curtain result is still eight times above the scientific recommendation.
• A 2022 Northwestern University study found even 100 lux during sleep raised heart rate and insulin resistance within a single night.
• Children are up to twice as sensitive to nighttime light as adults. Even 5-40 lux suppresses melatonin by 78% in preschoolers (Hartstein et al. 2022).
• Sealed-track design does not produce a low lux reading. It eliminates the conditions for any lux reading at all.

Ready for true 100% blackout? Explore the Ultimate Blackout Shade.

Frequently asked questions

What does 0 lux mean in a bedroom?

Zero lux means no measurable light is reaching the room. The Brown et al. 2022 consensus paper (PLOS Biology) recommends a maximum of 1 lux during sleep to support circadian function and melatonin production. A sealed-track system achieves 0 lux not by blocking light more effectively than other fabrics, but by eliminating the gaps through which light would otherwise enter.

Why do blackout curtains let light in even with thick fabric?

Blackout curtains block light through the fabric but cannot seal the gaps where fabric meets the wall. Top, side, and bottom gaps are structural failure points present in any hanging system regardless of fabric weight. A standard 57-inch curtain creates approximately 60 square inches of unobstructed gap area that fabric density cannot address.

What is the minimum lux level that affects sleep?

The Brown et al. 2022 consensus recommends a maximum of 1 lux during sleep. Controlled studies have measured melatonin suppression at levels as low as 5-10 lux in adults, with children showing sensitivity at the lower end of that range.

Do blackout roller shades without side channels work?

They reduce light significantly but do not reach the 1 lux sleep threshold. Customer reviews for inside-mount blackout roller shades without side channels consistently report 4-22 lux at room center. That falls below melatonin suppression levels documented in most adult studies but remains above the Brown et al. recommendation.

How is a sealed-track shade different from a regular blackout shade?

A regular blackout shade hangs in front of a window, leaving edge gaps regardless of fabric density. A sealed-track shade feeds into rigid aluminum channels on both sides, with a sealed head rail at the top and a sill channel at the bottom. This four-sided mechanical seal eliminates all gaps. The result is not a better lux reading. It is no lux reading.

Is 100% blackout worth it over 99% blackout?

The difference is not 1%. At typical morning exterior illuminance, 99% blackout still admits enough light to exceed the Brown et al. 1 lux sleep recommendation. A sealed-track system meets that threshold by design. No 99% fabric-based solution does, because remaining leakage enters through edge gaps, not the fabric itself.