In March 2023 I was reviewing a bedroom in Beaverton, west of Portland, where the owner had replaced blackout curtains that had worked for three years. The new curtains were labeled “blackout” but within two weeks she was waking earlier and sleeping shallowly. The difference was 3/4 of an inch: the old curtains overlapped the wall mount by 2 inches on each side; the new ones sat flush with the window frame. A 3/4-inch gap at 6 am in northwest Oregon June — civil twilight before 5:30 am — produces two vertical strips of full daylight on the wall across the room. The body responds whether or not the sleeper consciously notices it.
What light does during sleep
Light during sleep fragments sleep architecture even when it does not fully wake a sleeper. The National Institutes of Health has published research documenting that low-level illuminance during sleep affects the proportion of slow-wave sleep. A bedroom allowing 10 to 20 lux in the hour before natural wake time is likely disrupting the last sleep cycle. A 6 am June sky in the Pacific Northwest delivers 5,000 to 10,000 lux at the window — even 99-percent-effective blackout fabric lets 50 to 100 lux through. Gap light at the curtain edges delivers full daylight with no filtering at all.
What makes a blackout shade or curtain actually work
The three gaps in a typical curtain installation are: at the top (between the rod and the wall or window frame), at the sides (between the curtain edge and the wall), and at the bottom (between the fabric hem and the sill or floor). Each gap can deliver more light than the rest of the fabric combined, because daylight intensity is roughly 100 to 1,000 times higher than indoor artificial light.
- Side overlap: Curtain rods that extend 6 to 8 inches beyond the window frame on each side, with curtains long enough to cover that extension when closed, eliminate side-gap light. The curtain face should overlap the wall by at least 3 to 4 inches at the edges when drawn.
- Top seal: A valance, ceiling mount, or a blackout curtain track system that attaches flush to the ceiling eliminates top-gap light. A ceiling-mounted track costs $60 to $150 for a standard window and mounts flush, eliminating all top-gap exposure.
- Bottom drop: A blackout curtain that ends at the floor — not at the sill — eliminates bottom-gap light from an exterior light source at low angle or from a covered porch light. For windows above grade where light comes in at a downward angle, a curtain to 1 inch above the floor is adequate.
- Blackout roller shades inside the window frame: A dual shade system — a blackout roller inside the frame and a separate curtain outside — gives complete coverage with no gaps and allows the curtain to be decorative rather than functional. Blackout roller shades sized to fit the window opening with side channels cost $80 to $200 per window installed, or $30 to $80 as a DIY kit.
Dimmers and the order of priority
High illuminance in the 30 to 60 minutes before sleep delays melatonin rise. Dimming overhead lights to 10 to 20 percent output — roughly 20 to 50 lux at floor level — is the practical goal in the final hour before bed. Dimmer switches compatible with LED bulbs cost $25 to $55; the common failure is pairing a dimmer with a non-dimmable LED, which causes flickering at low settings.
If forced to choose between eliminating blackout gaps and installing dimmers, eliminate the gaps first. A well-sealed blackout shade with overhead lights at full output until bedtime is a better environment than a perfectly dimmed room with 3/4-inch curtain gaps letting in June dawn at 5 am. Morning light has the largest impact on when the next sleep cycle starts.
For the overall ventilation side of the bedroom equation — what the air is doing while the blackout shade is doing its job — the article on bedroom air at night covers CO2 accumulation and ventilation rates in a closed room overnight, which is a different but equally concrete parameter to measure and correct.
Frequently asked questions
How effective do blackout curtains have to be to make a difference in sleep? Complete blackout — below 1 lux — is the target for the sleep environment per NIH sleep research. Most curtains labeled “blackout” achieve 99 percent or more light reduction through the fabric but fail at the edges. The gap light, not the fabric transmission, is usually the problem. A curtain that overlaps the wall by 4 inches on each side and covers the full top and bottom eliminates 95 percent of gap-related light intrusion.
What lux level should a bedroom be at night for sleep? Below 5 lux at pillow level is the recommended threshold for uninterrupted sleep. A digital lux meter costs $20 to $40 and provides an exact reading. Most modern smartphones can also estimate lux using the ambient light sensor with a free app, accurate enough for this purpose.
Are blackout roller shades better than blackout curtains? Roller shades mounted inside the window frame with side-channel tracks eliminate more gap light than most curtain systems. They are more consistent and require less setup. The trade-off is that a tight-fitting roller shade with side channels typically costs $80 to $200 per window installed, versus $30 to $80 for a good blackout curtain panel. Many people use both — a blackout roller for the light control function and a curtain panel over it for aesthetics.