In a two-experiment study of 127 young adults, wearing an eye mask while sleeping improved performance on a memory task and a sustained-attention test the next day. The effect correlated with time spent in slow-wave sleep.
Light leaking into a bedroom — from streetlamps, device LEDs, or early sunrise — is a common and often overlooked sleep disruptor. A study from Cardiff University tested whether simply blocking that light with an eye mask would produce measurable cognitive benefits the next day.
The Study Design
Greco et al. ran two experiments, published in SLEEP, both using within-subjects designs where each participant served as their own control.
Experiment 1 (94 participants, ages 18-35): Participants wore an eye mask while sleeping for five nights, then slept without one for five nights (or vice versa, counterbalanced). On the morning after each block, they completed two tasks — a paired associates learning task (PAL), which tests memory encoding, and a psychomotor vigilance task (PVT), which measures sustained attention and reaction time.
Experiment 2 (33 participants, ages 18-35): Same protocol, but with polysomnography (EEG) on the final night of each condition to measure sleep architecture directly.
What Changed — and What Did Not
Wearing the eye mask improved performance on both cognitive tasks. PAL scores were higher in the mask condition, indicating better next-day memory encoding. PVT reaction times were faster, indicating sharper sustained attention.
What the mask did not change: total sleep time, self-reported sleep quality, and overall sleep architecture. Participants slept the same amount and rated their sleep similarly whether they wore the mask or not.
However, EEG data from Experiment 2 revealed that time spent in slow-wave sleep correlated with the memory improvement in the mask condition. The mask appears to have improved the quality of deep sleep by eliminating ambient light interference, without changing total sleep duration.
One notable null finding: the mask had no effect on motor skill learning (measured by a finger-tapping sequence task). The benefit was specific to declarative memory and attention, not procedural memory.
Why Light Matters Even Through Closed Eyelids
Eyelids block roughly 85-90% of incoming light, but the remaining 10-15% is enough to affect sleep physiology. Retinal ganglion cells that regulate circadian timing respond to light even when eyes are closed, and ambient light exposure during sleep has been linked to shallower sleep stages, reduced melatonin secretion, and increased heart rate during the night.
The mask eliminates this residual exposure entirely. It is a cheaper and simpler intervention than blackout curtains and works in any environment — travel, a partner who reads in bed, or a bedroom with imperfect light control.
Limitations
The study enrolled healthy young adults (18-35) who slept at home, not in a lab. The results may not apply to older adults, people with sleep disorders, or those who already sleep in completely dark rooms. The sample size was moderate — large enough to detect the effects found, but not large enough to identify smaller subgroup differences.
This is a single study, not a body of evidence. The findings are consistent with what we know about light and sleep physiology, but replication in larger and more diverse samples would strengthen the case.
Practical Application
An eye mask is low-cost, low-risk, and easy to test. If your bedroom is not completely dark — and most are not — wearing one is a straightforward way to reduce ambient light exposure during sleep. Based on this study, the cognitive benefits may be most relevant for tasks that depend on memory encoding and sustained attention: studying, detail-oriented work, or anything that requires focus the next morning.
Jessica Williams
