The body reads darkness as the signal of night, and a hotel room rarely sends it.

Why it matters

An advanced guest notices it within the first hour: the television's standby light, the bright line under the door, the smoke detector, the street lamp seeping in at the edge of the curtain. They do not always say so. They note it, and they compare.

Making a room truly dark is one of the least expensive interventions a hotel can make. It is also one of the easiest to measure.

What science establishes

Evening light shifts and shortens the biological night. In a laboratory study of 116 adults, ordinary room light (under 200 lux) in the hours before bedtime delayed the onset of natural melatonin secretion in 99 % of participants and shortened its duration by about 90 minutes, compared with dim light (under 3 lux). Gooley et al., 2011. Established.

A single night of moderate light during sleep can be measured the next day. In healthy adults, sleeping one night under 100 lux, instead of under 3 lux, increased night-time heart rate, lowered heart rate variability and increased insulin resistance the following morning. Mason et al., 2022. Established in the laboratory, over one night and in a small sample.

Blocking light works. In a trial in a simulated environment, wearing an eye mask and earplugs lengthened REM sleep, reduced micro-arousals and increased night-time melatonin. Hu et al., 2010. On ordinary nights, wearing a mask for a week improved next-day alertness and memory in young adults. Greco et al., 2023. Established, modest effects, young populations.

These studies mostly involve young, healthy adults; the effect in older guests is plausible, not measured here.

What is emerging

Large cohorts associate light in the bedroom with health markers. Among 43,722 women followed for nearly six years, sleeping with a light or a television on in the room is associated with more frequent weight gain. Park et al., 2019.

In Japan, the HEIJO-KYO cohort measured the light in 2,947 bedrooms with a lux meter. The median is 1 lux. The brightest quarter is associated with higher weight, waist circumference and LDL cholesterol; above 3 lux, with more sleep disturbance and depressive symptoms. Obayashi et al., 2022. In the same cohort, an average light of 5 lux or more is associated with more diabetes appearing during follow-up. Obayashi et al., 2019.

These are associations, not proof of cause. Yet they converge on the same order of magnitude: a few lux are enough to make the difference between a dark room and one that is not.

What we refuse to say

We do not say that a dark room makes you lose weight, prevents diabetes or lengthens life. No study shows it.

We do not say that a night in the dark at a hotel "repairs" anything. We say that it removes a stray signal that science knows how to measure.

We use no miracle threshold. The 3 and 5 lux thresholds cited above are those of the studies, not standards.

What a hotel can do

Why the room is almost never dark. Three leaks recur in almost every room: the edges and top of the curtains, the gap under the door, and the standby lights of devices (television, air conditioning, detector, USB sockets, minibar, alarm clock). Each seems negligible. Together, they often put the room above one lux.

Minimum baseline (a few tens to a few hundred euros per room). A quality eye mask, offered or available; blackout stickers on the lights visible from the bed; a door-bottom seal; an alarm clock with no lit display, or one that can be switched off; a low amber night light at floor level, motion-activated, to get up without switching on the lights.

Optimised baseline (around 800 to 1,500 € per room, depending on the windows). Total blackout with side channels and a top box; a "night" switch at the bedside that turns off all lighting in one gesture; safety signage that is compliant but angled so as not to light the bed.

The target. A room close to 0 lux at pillow level, lights off, measured and published. These orders of magnitude are indicative: they depend on the windows, local safety standards and the installer's quote.

What the Living Lab measures

Measurement planned. The Living Lab, near Lisbon, is being equipped in waves. Night-time light at pillow level, lights off, before and after blackout, will be the subject of a dated reading published here.

The protocol is simple: a lux meter placed on the pillow, three readings per night, over several nights. It is the same protocol as the one used in an audit.

Sources

• Gooley JJ et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab, 2011. doi:10.1210/jc.2010-2098
• Mason IC et al. Light exposure during sleep impairs cardiometabolic function. PNAS, 2022. doi:10.1073/pnas.2113290119
• Hu RF et al. Effects of earplugs and eye masks on nocturnal sleep, melatonin and cortisol in a simulated intensive care unit environment. Crit Care, 2010. doi:10.1186/cc8965
• Greco V et al. Wearing an eye mask during overnight sleep improves episodic learning and alertness. Sleep, 2023. doi:10.1093/sleep/zsac305
• Park YM et al. Association of exposure to artificial light at night while sleeping with risk of obesity in women. JAMA Intern Med, 2019. doi:10.1001/jamainternmed.2019.0571
• Obayashi K et al. Associations between indoor light pollution and unhealthy outcomes in 2,947 adults (HEIJO-KYO). Environ Res, 2022. doi:10.1016/j.envres.2022.114350
• Obayashi K et al. Bedroom lighting environment and incident diabetes mellitus (HEIJO-KYO). Sleep Med, 2019. doi:10.1016/j.sleep.2019.07.006

The longevity audit for venues  ·  The Research Journal

IO8 Living Lab, near Lisbon. Measured, never promised.

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