What it is
Blackout curtains, liners and shades use a dense weave or a coated backing to block light. How dark the room gets depends as much on fitting as on fabric: light leaks round the sides, top and bottom, so wide panels, rods mounted above and beyond the frame, and shades fitted inside the recess with side channels make the difference.
Many also cut draughts, and 'thermal' versions reduce heat gain and loss at the window.
How strong is the evidencelimited
No trials test curtains themselves, but the case for darkness is supported in the lab: one night of sleep with moderate room light (100 lux) raised night-time heart rate and next-morning insulin resistance in healthy adults compared with dim light (under 3 lux). In 552 older adults, any light during the darkest hours was linked with more obesity, diabetes and hypertension, an observational finding that can't prove cause.
The claims you’ll meet4 checked
Blocking outside light helps you sleepHolds upLight at night suppresses melatonin and can wake you, especially at a summer dawn. The evidence is mechanistic and lab-based rather than trials of blinds.
Sleeping in the dark protects metabolic healthMixedA one-night lab study and observational data point the same way; long-term benefit hasn't been tested.
'100% blackout' means a pitch-dark roomMixedThe fabric may block all light, but gaps round the frame usually don't.
Blackout curtains block noiseMixedHeavy fabric softens some higher-pitched sound; it won't silence traffic.
What to look for4
- ✓Coverage: panels wider and longer than the window, mounted above and beyond the frame, or a shade fitted inside the recess with side channels.
- ✓A blackout lining or coated backing. Hold the fabric up to a bright light to check.
- ✓Cordless designs if children live in or visit the home.
- ✓Washable, or at least easy to clean. Heavy curtains collect dust.
What to skip2
- ×'Noise-cancelling' claims. Expect a small reduction, not silence.
- ×Paying extra for a '100% blackout' fabric when the fit will leak light anyway.
How to use it2
- →Open them, or get outside, soon after waking. Darkness at night works best alongside bright light in the morning.
- →Cover standby lights and chargers too.
Take care2
- !Looped cords on blinds and shades can strangle young children. Choose cordless or keep cords out of reach with tension devices.
- !A fully dark room makes night-time trips riskier, especially for older adults. Use a dim, warm night-light low down.
The studies2 cited · checked against PubMed
Light exposure during sleep impairs cardiometabolic function ↗Mason IC, Grimaldi D, Reid KJ, Warlick CD, Malkani RG, Abbott SM, Zee PC · Proceedings of the National Academy of Sciences · 2022Randomized lab study
In healthy adults, one night of sleep with moderate room light (100 lux) raised night-time heart rate, lowered heart rate variability and increased next-morning insulin resistance compared with sleep in dim light (under 3 lux).
Light at night in older age is associated with obesity, diabetes, and hypertension ↗Kim M, Vu TH, Maas MB, Braun RI, Wolf MS, Roenneberg T, et al. · Sleep · 2023Cross-sectional study
552 adults aged 63-84 wore light sensors for a week. Any light during the darkest 5 hours was linked with higher odds of obesity (1.82), diabetes (2.00) and hypertension (1.74); the design cannot show cause.