To fall asleep, the body has to lose heat, and a room that is too warm prevents it.

Why it matters

An advanced guest arrives with a precise habit: a cool room, a duvet, sometimes a window. They discover a thermostat that refuses to go below a value set by the hotel. It is one of the most frequent remarks, and one of the simplest to correct.

What science establishes

Core temperature falls at the start of the night, and falling asleep follows this drop. A review devoted to the thermal environment and sleep points this out: the regulation of temperature and that of sleep are linked. Okamoto-Mizuno and Mizuno, 2012. Established.

In real conditions, with bedding and clothing, it is heat that degrades sleep the most. It increases awakenings and reduces deep sleep and REM sleep. Humidity makes this effect worse. Cold, for its part, does not alter the stages of sleep as long as the bedding is sufficient. Same review. Established.

In older people, the range for the best sleep can be measured. One study followed older adults at home, with bedroom sensors and sleep sensors. Sleep was most efficient between 20 and 25 °C, and efficiency fell by 5 to 10 % when the room went from 25 to 30 °C. Differences between individuals were large. Baniassadi et al., 2023. Established for this population, at home, with their own bedding.

What is emerging

Why IO8 adopts 18 to 19 °C. This figure does not come from a single trial. It reflects a logic: cool air, combined with suitable bedding, gives the body the possibility of shedding its heat without feeling cold. It is a starting value, not a universal optimum.

The study in older people, with a range of 20 to 25 °C, does not contradict it: it measures the air in the room with each person's usual bedding. Above all, it shows that needs vary from one person to another. The consequence for a hotel is clear: the guest must be able to adjust their room.

The same review points out that cold, without altering the stages of sleep, can influence the night-time cardiac response. Okamoto-Mizuno and Mizuno, 2012. This point remains to be studied.

What we refuse to say

We do not say that sleeping in the cool makes you lose weight, activates "brown fat" or slows ageing.

We do not say that 18 °C is the right temperature for everyone. We say that a room must be able to go down to it.

What a hotel can do

Why thermostats are restricted. To control energy consumption, many hotels limit the adjustment range of their rooms, sometimes to a few degrees around a fixed set point. The guest cannot go down to where they sleep well.

Minimum baseline (a few tens to a few hundred euros per room). Widen the thermostat range downwards, at least to 18 °C; offer two duvets of different weights; breathable sheets (percale, linen); a silent fan on request; a readable thermometer in the room.

Optimised baseline. Silent air conditioning, adjustable degree by degree; controls that cool the room before the guest arrives; a bed temperature regulation system, lent for the night like the other devices in the IO8 stack.

The target. A room able to hold 18 to 19 °C at night, adjustable by the guest, with bedding to match. The amounts depend on the existing installation; widening a thermostat range often costs no more than a setting change.

What the Living Lab measures

Measurement planned. The Living Lab, near Lisbon, is being equipped in waves. The temperature and humidity of the bedroom will be recorded throughout the night, alongside a sleep sensor, then published here with their date.

The audit reading follows the same logic: a recorder near the bed, a curve over the night, and a check of the thermostat's actual range.

Sources

• Okamoto-Mizuno K, Mizuno K. Effects of thermal environment on sleep and circadian rhythm. J Physiol Anthropol, 2012. doi:10.1186/1880-6805-31-14
• Baniassadi A et al. Nighttime ambient temperature and sleep in community-dwelling older adults. Sci Total Environ, 2023. doi:10.1016/j.scitotenv.2023.165623

The longevity audit for venues  ·  The Research Journal

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

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