Sleeping with lights on: sleep time is not the whole story

What the overnight experiment actually compared

A March 2022 laboratory study randomized 20 healthy young adults into two groups. Ten slept under dim light, below 3 lux, for two nights. Ten had the same first night, followed by a night with overhead lighting at 100 lux. Researchers measured sleep using polysomnography. (Mason and colleagues: overnight light experiment, 2022)

The groups differed in their night-to-night changes in sleep-stage proportions. For slow-wave sleep, often called deep sleep, the room-light group showed a smaller increase than the dim-light group. The percentage rose in both groups, by different amounts. Changes in total sleep time and sleep fragmentation did not differ significantly. (Mason and colleagues: overnight light experiment, 2022)

The experiment was small and the brighter exposure lasted one night. It cannot predict your response, establish a safe brightness threshold or prove that a particular curtain increases deep sleep. Funding included Northwestern University and NIH support; the authors declared no competing interest. (Mason and colleagues: overnight light experiment, 2022)

For a shopper, the useful question is broader than whether you remember waking up: what outcome was actually measured? Sleep duration, sleep stages and a personal impression of rest are different observations. A product promising better sleep should explain which one it tested.

Evening lighting and sleeping conditions have different jobs

A separate March 2022 expert consensus recommends brighter daytime conditions, lower evening light and a sleep environment as dark as possible. It explicitly allows for nighttime activities that require vision. It primarily addresses healthy adults with regular daytime schedules; it is not a bedroom-product trial. (Brown and colleagues: light-exposure consensus, 2022)

Its targets use melanopic equivalent daylight illuminance, which weights light for its influence on melanopsin-sensitive retinal cells. That is different from ordinary illuminance in lux. Do not directly compare those targets with an unexplained phone-meter reading or assume a warm-looking bulb meets them. Light spectrum and measurement position matter. (Brown and colleagues: light-exposure consensus, 2022)

The workshop received BBSRC and University of Manchester support. Some consensus authors disclosed lighting-industry relationships or relevant patents. The paper is useful for understanding measurement and timing, but it does not certify a lamp, mask or mattress as a sleep treatment. (Brown and colleagues: light-exposure consensus, 2022)

A practical approach is to make two settings: enough light for your evening task, then a separate arrangement for sleeping. Someone reading beside a sleeping partner has a different problem from someone whose window faces a streetlight. Name the problem before buying an accessory.

Check the room from where you actually sleep

NIOSH's sleep-environment guidance, reviewed in March 2020, recommends opaque window coverings, addressing light around their edges and reducing light entering through doorways. It also suggests an eye mask when other measures are insufficient and a dim red nightlight for seeing the route to the bathroom. (NIOSH: creating a good sleep environment)

Use that guidance as a starting point for a simple inspection, not a demand to replace everything in the bedroom: (NIOSH: creating a good sleep environment)

• Look from pillow height after your usual lights-out routine. Identify the window, doorway, lamp or display responsible for unwanted light.
• Check the edges of existing curtains before deciding that a different fabric is necessary.
• If considering a mask, compare fit and comfort in your normal sleeping position rather than relying on a deep-sleep claim.
• Keep needed navigation lighting and a clear walking route. Darkness should not make getting up unsafe.

Choose a change you can comfortably maintain

Start with one identifiable source of unwanted light. Make a brief note of what you changed and whether the arrangement is comfortable and practical over several nights. Those notes can guide your preference, but they do not reproduce a sleep laboratory or establish a cause for every good or bad night.

A better mattress addresses a different part of the bedroom than a glowing hallway or uncovered window. Treat lighting as its own comfort decision. The aim is a workable sleep environment; no single accessory can promise a particular amount of deep sleep.