Experiment guides → Bedroom & environment → Darkness
Does sleeping in total darkness improve sleep?
Quick answer: the evidence for darkness itself is stronger than most people expect. In a controlled study, one night of sleeping under a modest 100 lux — a dim lamp, or streetlight through thin curtains — raised heart rate, lowered heart-rate variability and increased next-morning insulin resistance, even though people slept. Population data from Japan link bedroom light at night to sleep disturbance and lower melatonin. And in two experiments, simply wearing an eye mask improved next-day memory and alertness. Darkness costs a set of blinds or a mask, and 14 nights will show whether your mornings notice.
"I will sleep in complete darkness — blackout blinds or an eye mask — for 14 nights."
What the research says
One night of dim light: measurable physiological cost. Mason and colleagues had 20 young adults sleep one night in near-darkness (3 lux) and one under 100 lux overhead light. Under light, night-time heart rate rose, heart-rate variability fell, and insulin resistance the next morning was higher — while sleep duration barely changed. The body registered the light even though the sleeper didn't (Mason et al., 2022).
In real bedrooms, light at night tracks poorer sleep. In the HEIJO-KYO cohort of older Japanese adults, bedroom light measured overnight — especially short-wavelength light — was associated with sleep disturbances and lower overnight melatonin. Cross-sectional, so it can't prove cause, but it's the same direction as the lab, in ordinary homes (Mitsui et al., 2022).
An eye mask alone improved the next day. Greco and colleagues ran two experiments (89 and 33 participants) in which people wore an eye mask during sleep for a week and went without for another. With the mask, they learned word pairs better and were more alert on a vigilance task the next day, with the alertness gain linked to more slow-wave sleep on the mask nights. Cheap intervention, measured cognitive outcome (Greco et al., 2023).
What this evidence doesn't say
The lab study was one night in 20 people; whether the metabolic effect persists or adapts over weeks is unknown. The cohort data are observational. The eye-mask experiments measured memory and alertness, not subjective sleep quality. And total darkness has a flip side: morning light is your clock's main cue, so blackout blinds that stay shut past wake time can push your rhythm later.
Run it as an experiment
- Baseline five nights, and measure the light. Note where the light comes from — streetlamp, a charger LED, the hallway. A phone light-meter app at pillow level is good enough: anything above a few lux is worth blocking.
- Choose blinds or a mask. Blackout blinds fix the room; a contoured eye mask fixes the eyes and travels. A mask is the cheaper test and the one with a trial behind it — choose one that doesn't press on the eyelids.
- Make the pact. "I will sleep in complete darkness for 14 nights." Cover or unplug the LEDs, and keep the phone face-down or out of the room.
- Open the curtains the moment you wake. Darkness for sleep, light for waking. If you use blackout blinds, raising them within minutes of the alarm protects your morning clock signal.
- Compare restedness and morning alertness. Those are the outcomes with evidence behind them. Better mornings? Keep it. No change? Your room was probably dark enough already — a good thing to know before buying blinds.
What to expect
If your room was letting in light, the change is often subtle at night and clearer in the morning: waking feels less foggy, and the alertness gain the trial measured is the kind you notice by mid-morning. If the room was already dark, expect no change. A mask takes a few nights to stop noticing; the first two mornings are not the verdict.
Who should skip this
If you need to see a child, a patient or a hazard in the night, keep a very dim, warm-coloured night light rather than full darkness. People with anxiety about darkness shouldn't force it. Otherwise there's no meaningful risk — but don't let blackout blinds delay your morning light.
Frequently asked questions
Do blackout curtains improve sleep quality?
Where light is getting in, the physiology says yes: even dim light during sleep raised heart rate and morning insulin resistance in a controlled study. Where the room is already dark, blinds add nothing. Measure first, then decide.
Does wearing a sleep mask help?
Two experiments found a week of eye-mask nights improved next-day learning and alertness compared with mask-free nights, with more slow-wave sleep on mask nights. It's the cheapest and best-tested version of the darkness experiment.
Can a room be too dark for sleep?
Not for sleep itself. The only cost is missing the morning light your body clock uses to set itself — solved by opening the blinds or taking the mask off as soon as you wake.
Related experiments
Sources
- Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci U S A. 2022;119(12):e2113290119. PubMed 35286195
- Mitsui K, Saeki K, Tone N, et al. Short-wavelength light exposure at night and sleep disturbances accompanied by decreased melatonin secretion in real-life settings: a cross-sectional study of the HEIJO-KYO cohort. Sleep Med. 2022;90:192–198. PubMed 35190318
- Greco V, Bergamo D, Cuoccio P, Konkoly KR, Muñoz Lombardo K, Lewis PA. Wearing an eye mask during overnight sleep improves episodic learning and alertness. Sleep. 2023;46(3):zsac305. PubMed 36521010
Track this experiment in N of 1
A few baseline nights, then the change — and your own mornings deliver the result. No streaks, no guilt. Your first experiment is free.
Downloadon the App StoreThis page is general information about published research, not medical advice. If you have a sleep disorder, are pregnant, or take medication, talk to your doctor before changing your routine.