The body clock is set by light, and a hotel room often sends it the wrong signal at the wrong time.

Why it matters

In the evening, a cold white ceiling light, a bathroom mirror lit like a make-up booth. In the morning, curtains that stay closed and a room that never rises above a few tens of lux. The advanced guest knows the rule: bright light in the morning, warm and low light in the evening. They quickly see whether the room follows it.

The night itself, lights off, is covered in the article on total darkness (01). Here, we are talking about the two edges of the night.

What science establishes

A dedicated sensor in the eye. Certain cells of the retina, the melanopsin ganglion cells, are sensitive to light in their own right and transmit this signal to the brain's central clock. Berson et al., 2002, in rats. They are most sensitive to blue, short-wavelength light. Established.

The timing of light decides the direction of the shift. In 21 adults in the laboratory, the same bright light advances the clock when it comes after the low point of body temperature, at the end of the night and in the morning, and delays it when it comes before, in the evening. Khalsa et al., 2003. Established.

Screen light in the evening can be measured the next day. Reading for four hours on a backlit e-reader before bedtime, instead of a printed book, delayed falling asleep, reduced natural melatonin secretion, shifted the clock and reduced alertness the next morning. Chang et al., 2015, in young adults in the laboratory. Established.

An international consensus puts figures on the thresholds. A group of researchers recommends, measured at eye level, at least 250 melanopic lux during the day, at most 10 melanopic lux in the three hours before bedtime, and at most 1 lux during sleep. Brown et al., 2022. The unit, known as “melanopic illuminance”, takes the colour of the light into account: for a warm white LED of 2,700 to 3,000 K, whose melanopic ratio is about 0.4 to 0.5, the evening limit would correspond to an illuminance of around 20 to 25 lux at eye level. Established, in adults aged 18 to 55.

What is emerging

After 55, the eye receives less. The lens yellows with age. For the same light, the dose received by the retina can be reduced by about half compared with a young adult. Brown et al., 2022, conclude that older people may need more light during the day. Emerging.

A room lit according to the time of day. In a controlled trial of 196 cardiology patients with an average age of 66, a room with dynamic lighting (bright light for two hours during the day, dim at night) lengthened sleep measured at the wrist by about 29 minutes after five days, without changing perceived sleep. Giménez et al., 2017. Emerging, in hospital, not in a hotel.

Older people lack light. In a Scottish care home, 16 residents aged 72 to 99 spent on average 46 minutes a day above 1,000 lux in summer, and 3 minutes in winter. Nioi et al., 2017. Emerging, small sample.

What we refuse to say

We do not say that morning light cures insomnia or depression. It helps the clock to set itself; that is what the studies measure.

We do not say that a 2,700 K bulb is enough. What counts is the amount of light that reaches the eye: a warm but intense light remains a daytime signal.

We do not say that filtering glasses, apps or a “night” mode replace a well-lit room.

What a hotel can do

Why the room gets it wrong. Hotel lighting is designed for the image: white, uniform, generous. It is as bright at 11 pm as at 6 pm, and rarely bright enough in the morning.

Minimum baseline (around 100 to 300 € per room). Bulbs of 2,700 K or less for all evening sources; dimmers on the bedside lamps and the ceiling light; a bedside lamp pointing downwards, not towards the eyes; curtains that are easy to open wide, and a note inviting guests to do so on waking.

Optimised baseline (around 800 to 2,000 € per room). Controlled lighting that moves from bright white light in the morning to warm, low light in the evening; a wake-up lamp that exceeds 1,000 lux at eye level; a breakfast table or reading corner near the window.

The target. More than 1,000 lux in the morning and less than 2,700 K in the evening, measured and published for each room. These orders of magnitude are indicative, to be confirmed by quote.

What the Living Lab measures

Measurement planned. The Living Lab, near Lisbon, is being equipped in waves. Illuminance and colour temperature in the room will be measured at eye level in the morning on waking and in the evening in the three hours before bedtime, with curtains open and then closed.

The readings will be published here, dated.

Sources

• Berson DM et al. Phototransduction by retinal ganglion cells that set the circadian clock. Science, 2002. doi:10.1126/science.1067262
• Khalsa SBS et al. A phase response curve to single bright light pulses in human subjects. J Physiol, 2003. doi:10.1113/jphysiol.2003.040477
• Chang AM et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS, 2015. doi:10.1073/pnas.1418490112
• Brown TM et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biol, 2022. doi:10.1371/journal.pbio.3001571
• Giménez MC et al. Patient room lighting influences on sleep, appraisal and mood in hospitalized people. J Sleep Res, 2017. doi:10.1111/jsr.12470
• Nioi A et al. Seasonal differences in light exposure and the associations with health and well-being in older adults: an exploratory study. HERD, 2017. doi:10.1177/1937586717697650

The longevity audit for venues  ·  The Research Journal

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

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