Effects of Humid Heat Exposure on Human Sleep Stages and Body Temperature
Study by K. Okamoto-Mizuno, K. Mizuno, S. Michie, A. Maeda and S. Iizuka (Dokkyo University School of Medicine, Japan)
Sleep Lab Study of 7 Adults · Sleep (Vol. 22, Issue 6, pp. 767-773) · DOI: 10.1093/sleep/22.6.767
Key Takeaway
Hot, Humid Nights Meant More Waking and Less Deep Sleep
What was tested: 7 healthy young men slept in a lab under 4 room conditions: 29°C or 35°C, each at 50% or 75% relative humidity. They slept on a bed covered with a cotton sheet while their brain waves and body temperature were recorded (Okamoto-Mizuno et al., 1999).
What the numbers showed: The hottest, most humid condition (35°C and 75% humidity) caused more waking, lower sleep efficiency and less deep sleep than the 29°C conditions. REM sleep also fell. Body core temperature stayed higher than in the other conditions.
What this means for you: Heat and humidity together can make sleep lighter and more broken. The study tested room air, not mattresses, so it does not show that any mattress or cover keeps you cooler.
Key Numbers
How the Study Was Done
Researchers in Japan studied 7 healthy male volunteers with an average age of about 23. Each slept from 11 pm to 7 am under 4 conditions of room temperature and relative humidity: 29°C at 50%, 29°C at 75%, 35°C at 50% and 35°C at 75% (Okamoto-Mizuno et al., 1999).
The men wore only shorts and slept on a bed covered with a 100% cotton sheet. Brain waves, eye movements and chin muscle activity were recorded all night to score sleep stages. Rectal (core) temperature and skin temperature were measured continuously.
What the Study Compared
29°C Room, 50% or 75% Humidity
The cooler conditions. More deep sleep and higher sleep efficiency than the hot, humid condition.
35°C Room, 75% Humidity
More time awake, lower sleep efficiency, less deep sleep and less REM sleep. Core temperature stayed higher.
What the Study Found
At 35°C and 75% humidity, the men spent more time awake and had a lower sleep efficiency and less deep sleep (stages 3 and 4) than at 29°C with either humidity. REM sleep and stage 3 were significantly lower at 35°C and 75% than at 29°C and 50% and at 35°C and 50% (Okamoto-Mizuno et al., 1999).
Core temperature stayed higher at 35°C and 75% than in the other conditions, and skin temperature was higher in both 35°C conditions. The authors conclude that humid heat at night adds to the body's heat load, blocks the normal fall in core temperature during sleep, reduces deep and REM sleep, and increases waking.
An earlier version of this page cited a 2021 study whose DOI belongs to an unrelated paper. We could not find that study, so its in-bed temperature and humidity figures have been removed. The abstract we read does not state who funded this study.
How to Apply This Study When Choosing a Mattress
What it tells you: Keeping the bedroom from getting hot and humid helps protect deep sleep.
This study changed room air, not bedding. A breathable cover or cooler bedding may help you feel cooler, but this study does not test any mattress, so it cannot tell you which one sleeps coolest.
Study Scope and Limitations
7 healthy young men, room conditions set in a lab, and no mattress comparison. The hot conditions (29 to 35°C) are warmer than most bedrooms. Funding is not stated in the abstract.
Primary Citation
Okamoto-Mizuno, K., Mizuno, K., Michie, S., Maeda, A., & Iizuka, S. (1999). Effects of humid heat exposure on human sleep stages and body temperature. Sleep, 22(6), 767-773. https://doi.org/10.1093/sleep/22.6.767