Darkness and eye masks

Make the bedroom as dark as you can, with blackout curtains or an eye mask. Experts recommend under 1 lux at the eye while you sleep.

• Evidence: Some
• Low effort
• Low cost
• Works: tonight

Why it matters

Light reaches special cells in your eye that tell your body clock whether it’s day or night. These cells don’t need much light to respond. In 2022, a group of light and sleep researchers agreed on targets for healthy adults: the sleep environment should be as dark as possible, with no more than 1 lux at the eye, measured in a unit called melanopic equivalent daylight illuminance (melanopic EDI) [1]. That is very dim.

Light during sleep may matter even when it doesn’t wake you. In a randomized lab study of 20 healthy adults, one night with room lighting at 100 lux, compared with dim light under 3 lux, gave a higher heart rate and lower heart rate variability during sleep, a larger share of light sleep, less deep and REM sleep, and more insulin resistance the next morning [2]. That’s one night, in a lab, with fairly bright light, but the direction is clear.

How to do it

1. Block outside light. Blackout curtains or blinds, ideally with side gaps covered. The consensus authors think many bedrooms are slightly above 1 lux and name blackout blinds as a simple fix [1].
2. Turn off or cover small lights. Standby LEDs, chargers, clocks and smoke-alarm lights add up. Use tape or a small sticker, or turn the device to face the wall. We found no study measuring how much standby LEDs alone affect sleep, so treat this as tidying up, not a big win.
3. Use an eye mask if you can’t make the room dark. This includes light summer mornings, a partner reading, travel, or city light you can’t block. Look for a contoured mask that doesn’t press on your eyes and has a strap that stays put without being tight.
4. Keep night-time lights dim. If you need light to get to the bathroom, use a dim, warm night light rather than the main light. The consensus suggests keeping necessary night-time light below 10 lux melanopic EDI at the eye [1].
5. Keep your phone face down or out of reach. Its screen lights up with notifications.

Common mistakes

• Thinking it only matters if light wakes you. In the lab study, people slept through the light, but their heart and sleep stages still responded [2].
• Giving up on the mask after one night. In the eye mask trial, people had 5 nights to get used to it before testing [3].
• A tight mask. Pressure on the eyes is uncomfortable. Pick one with cups or padding that sits away from the eyelids.
• Making the room dark but leaving the evening bright. The same consensus recommends no more than 10 lux melanopic EDI at the eye for the 3 hours before bed [1]. See screens and blue light.

Effect on your sleep score

• Heart rate and HRV: in the lab study, sleeping with 100 lux of light raised heart rate and lowered HRV compared with sleeping in dim light [2]. If your room is bright at night, darkening it may show up here, but we found no study measuring that on a consumer wearable.
• Deep sleep and REM: light during sleep reduced both in the lab study [2]. In a second experiment with 35 young adults wearing a headband EEG device, the memory benefit of the mask was linked to time spent in deep sleep [3].
• Total sleep: in the eye mask trial, sleep diaries showed about the same hours slept with and without the mask [3].

How confident: moderate that light during sleep affects heart rate and sleep stages; low on how much a mask changes your score at home.

Evidence

Rated Some. The 1 lux target is an expert consensus based on how sensitive the body clock is to light [1], not a trial of bedroom darkness. Direct evidence comes from small studies. The 100 lux lab study had 20 people and one test night [2]. In the eye mask trial, 89 adults aged 18–35 in the UK slept a week with a mask and a week without; with the mask they did better on a memory test and an alertness test [3]. In a study of 14 healthy people exposed to recorded intensive-care noise and light, earplugs plus eye masks gave more REM sleep and fewer arousals [4], but that tested both at once, under unusual conditions.

General information for healthy adults, not medical advice. If you have a sleep problem that lasts, read when to see a doctor. Sources checked 4 October 2026. Spot a mistake? Tell us.