Sleep Masks and Darkness: What Light at Night Does to Your Sleep

Most of us treat bedroom light as a comfort question. Researchers treat it as an exposure. Closed eyelids do not block everything, and the systems that regulate arousal and body clock timing keep responding to light overnight. Below is what the studies show, what they cannot show, and where an eye mask fits.

1. Closed eyelids are a weak filter

Your eyelids do some work, but not as much as people assume. In a 1996 study that used a visual threshold response, estimated light transmission through the human eyelid was 0.3 percent for blue light, 0.3 percent for green and 5.6 percent for red. The eyelid acted as a red-pass filter, so what gets through is skewed toward the red end of the spectrum.

That is one small study using a threshold method, not a measurement of your bedroom at 2am. It does explain why researchers keep asking what light during sleep does, rather than assuming closed eyes make it irrelevant.

Citation: Ando K, Kripke DF. Light attenuation by the human eyelid. Biol Psychiatry, 1996. PubMed: 8719122

2. What a lit room does to sleep in the lab

Two laboratory studies are worth knowing. In the first, ten healthy sleepers had polysomnography on two nights, one with the lights off and one with a bedside light on all night. Lights-on sleep was associated with more stage 1 sleep, less slow-wave sleep and a higher arousal index. Brain rhythms tied to sleep depth and stability, including slow oscillation, delta and spindle power, were lower in the lit condition.

Citation: Cho JR, Joo EY, Koo DL, Hong SB. Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms. Sleep Med, 2013. PubMed: 24210607

The second study compared one night of moderate light (100 lux) with a dim room (under 3 lux) in healthy adults. After the lit night, participants had a higher heart rate overnight, lower heart rate variability and higher insulin resistance the next morning. The authors read this as a sign of more sympathetic activation during light-exposed sleep.

Both studies are small and short. Ten people for one night each is a signal worth following up, not a verdict. And 100 lux is a lit room, so it says more about leaving an overhead or lamp on than about a faint standby LED.

Citation: Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci U S A, 2022. PubMed: 35286195

3. What home studies add, and what they cannot

The HEIJO-KYO cohort in Japan measured bedroom light with a meter in older adults. In 857 people, those in the brightest quartile of nighttime bedroom light had higher odds of subjective insomnia (adjusted odds ratio 1.61, 95% CI 1.05 to 2.45), and brighter bedrooms tracked with poorer actigraphic sleep, including lower sleep efficiency and more time awake after sleep onset.

Citation: Obayashi K, Saeki K, Kurumatani N. Association between light exposure at night and insomnia in the general elderly population: the HEIJO-KYO cohort. Chronobiol Int, 2014. PubMed: 25025617

A follow-up looked at 863 participants with no depressive symptoms at baseline. Over a median of 24 months, 73 developed them. Compared with bedrooms averaging under 5 lux (n = 710), bedrooms averaging 5 lux or more (n = 153) were associated with a higher risk of developing depressive symptoms (hazard ratio 1.89, 95% CI 1.13 to 3.14, and 1.72 after adjusting for sleep parameters and health factors).

Citation: Obayashi K, Saeki K, Kurumatani N. Bedroom light exposure at night and the incidence of depressive symptoms: a longitudinal study of the HEIJO-KYO cohort. Am J Epidemiol, 2018. PubMed: 28992236

These are real-world measurements, which the lab studies lack. They also have limits. The participants were older adults in one region, and the design is observational. People who sleep badly may leave lights on, and bedroom light can travel with other things such as shift work or room layout. Treat them as associations.

4. Do eye masks help? The hospital evidence

Most mask research comes from intensive care, where noise and light are hard to control. In a simulated ICU study, 14 healthy volunteers slept under recorded ICU noise and light. They had poorer perceived sleep quality, more light sleep and less REM sleep. Adding earplugs and eye masks was associated with more REM time, shorter REM latency, fewer arousals and higher melatonin levels (P = 0.002).

Citation: Hu RF, Jiang XY, Zeng YM, Chen XY, Zhang YH. Effects of earplugs and eye masks on nocturnal sleep, melatonin and cortisol in a simulated intensive care unit environment. Crit Care, 2010. PubMed: 20398302

A randomized trial in cardiac surgery patients enrolled 50 people and analyzed 45 (20 intervention, 25 control). Those who used earplugs and eye masks with 30 minutes of relaxing music rated their sleep better, including depth of sleep and overall quality. Urinary melatonin and cortisol did not differ between groups.

Citation: Hu RF, Jiang XY, Hegadoren KM, Zhang YH. Effects of earplugs and eye masks combined with relaxing music on sleep, melatonin and cortisol levels in ICU patients: a randomized controlled trial. Crit Care, 2015. PubMed: 25881268

Pulling it together, a Cochrane review included 30 trials and 1,569 participants across many non-drug sleep interventions, and rated the overall evidence low or very low. For earplugs or eye masks or both, two small meta-analyses of two studies each suggested a lower incidence of delirium (risk ratio 0.55, 95% CI 0.38 to 0.80, 177 participants) and about 2.19 more hours of total sleep time (95% CI 0.41 to 3.96, 116 participants). Both were rated low quality. The review also noted that most studies measuring objective sleep with earplugs and eye masks found no benefit.

Citation: Hu RF, Jiang XY, Chen J, et al. Non-pharmacological interventions for sleep promotion in the intensive care unit. Cochrane Database Syst Rev, 2015. PubMed: 26439374

5. Where the evidence is thin

Be careful with the gap between these studies and a healthy person in a bedroom. Nearly all the mask data is in hospitals or simulated hospitals, usually tested together with earplugs, music or both, so the mask cannot be separated from the rest. The light studies used lit versus dim rooms, so they do not tell us how much light is too much. The sample sizes are small, the home studies are observational, and we did not find a large randomized trial of eye masks in healthy adults sleeping at home.

An honest read is that light during sleep looks like a plausible, modest contributor to lighter, more fragmented sleep, and that a mask is a low-risk way to reduce it. That is a reasonable inference, not a proven result.

6. Practical ways to lower the light

You do not need a study to cover a bright standby light or close a gap in the curtains. The usual options are heavier or lined curtains, covering or turning off device LEDs, keeping hallway and bathroom lights low, and using a mask. If you wake at night, a dim, warm light low to the ground is gentler than an overhead lamp. We did not find research comparing these options with each other, so pick what you can stick with. This is general information, not medical advice. If you have ongoing trouble sleeping, talk to a clinician. For the daytime side of light and sleep, see our post on blue light and sleep.

Where Lunia fits

Lunia Rêve is a structured mulberry silk sleep mask. The three-dimensional shape and internal padding create space over the eyes instead of pressing on them, and a curved notch sits over the bridge of the nose. We will be plain about the limit: no mask gives 100 percent blackout, and the nose bridge is where most masks leak light. Rêve is built to reduce that gap, not to promise it away.

No trial has tested Rêve, and the studies above looked at hospital eye masks and room lighting, not our product. The research gives a reason to think about how much light reaches your eyes. A mask is one practical way to lower it.

Learn more about Lunia Rêve

Frequently Asked Questions

Does sleeping with a light on affect sleep?

In a small polysomnography study of ten healthy sleepers, whole-night bedside light was associated with more light sleep, less slow-wave sleep and a higher arousal index. Observational data in older adults point the same way, but they cannot show cause.

Do you need total darkness to sleep well?

The studies here compared a lit room with a dim one, so they do not identify a minimum safe level of light. In the home cohort, the group averaging 5 lux or more overnight had a higher risk of later depressive symptoms than the group below 5 lux. That is an association in older adults, not a rule.

Do eye masks actually improve sleep?

The best evidence comes from hospital settings. A Cochrane review found low-quality evidence that earplugs or eye masks, or both, may help total sleep time and delirium in ICU patients, while most studies measuring objective sleep found no benefit. Trials in healthy people at home are scarce.

Can a sleep mask block all light?

No mask can promise that. Light tends to leak at the nose bridge, and fit varies from face to face. A structured shape with a nose notch is built to reduce that gap, but a snug fit is a goal, not a guarantee.

Does light get through closed eyelids?

Yes, a little. A 1996 study estimated that about 0.3 percent of blue and green light and 5.6 percent of red light passes through the eyelid. That was a small threshold study, so treat the numbers as rough.

Is it safe to wear a sleep mask every night?

The trials we reviewed did not clearly report adverse effects, but reporting was patchy. Comfort and pressure on the face vary by person. If you have an eye condition or skin sensitivity around the eyes, ask a clinician before nightly use.

Is a mask better than blackout curtains?

No study we found compared them head to head. Curtains lower the light in the whole room, while a mask lowers what reaches your eyes. Some people use both, especially in bright cities or while travelling.

The Bottom Line

Light during sleep is associated with shallower, more broken sleep, and the lab results are consistent even though they are small. Eye masks have low-quality but supportive evidence from hospitals, and almost none from healthy people at home. If your bedroom is bright, reducing the light is a sensible, low-cost experiment. Just keep your expectations at the level of the evidence.

References

1. Ando K, Kripke DF. Light attenuation by the human eyelid. Biol Psychiatry, 1996. PubMed: 8719122
2. Cho JR, Joo EY, Koo DL, Hong SB. Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms. Sleep Med, 2013. PubMed: 24210607
3. Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci U S A, 2022. PubMed: 35286195
4. Obayashi K, Saeki K, Kurumatani N. Association between light exposure at night and insomnia in the general elderly population: the HEIJO-KYO cohort. Chronobiol Int, 2014. PubMed: 25025617
5. Obayashi K, Saeki K, Kurumatani N. Bedroom light exposure at night and the incidence of depressive symptoms: a longitudinal study of the HEIJO-KYO cohort. Am J Epidemiol, 2018. PubMed: 28992236
6. Hu RF, Jiang XY, Zeng YM, Chen XY, Zhang YH. Effects of earplugs and eye masks on nocturnal sleep, melatonin and cortisol in a simulated intensive care unit environment. Crit Care, 2010. PubMed: 20398302
7. Hu RF, Jiang XY, Hegadoren KM, Zhang YH. Effects of earplugs and eye masks combined with relaxing music on sleep, melatonin and cortisol levels in ICU patients: a randomized controlled trial. Crit Care, 2015. PubMed: 25881268
8. Hu RF, Jiang XY, Chen J, et al. Non-pharmacological interventions for sleep promotion in the intensive care unit. Cochrane Database Syst Rev, 2015. PubMed: 26439374

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.