Eight young adults and sixteen older adults, half of whom suffer from insomnia, spent two nights in a body-suit at a sleep laboratory (see image), with a night at home in between.
Water-filled micro-pipes in the suit maintained the skin temperature of the participants at either 35 degrees celsius in the cool condition or 35.4 degrees in the warm condition, fluctuating gradually between the two every 15 to 30 minutes. Importantly, core body temperature was unaffected by these subtle temperature fluctuations.
The controlled skin temperatures match the typical climate of a person’s bed and are close to the levels that people report to be of most comfort, with the warmer condition actually reported to be slightly less comfortable.
Recordings of the participants’ brain waves at night showed that warmer skin temperatures resulted in a shift in sleep depth towards deeper sleep and a reduction of their likelihood of being awake at 6am.
For instance, among the non-insomniac older participants, a subtle (only 0.4 degree) increase in skin temperature reduced the probability of being awake at 6am by a factor of 14; for those with a sleep problem, it was by a factor of five. Moreover, with the same subtle increase in temperature, the likelihood of an older insomniac participant being in a deep (slow wave) sleep was doubled for any point in the night.
The findings have huge practical implications, even before the development of user-friendly body-suits. For example, it is possible that the temperature environment people choose to sleep in, based on comfort, may not be optimal for inducing sleep.
A warm bath before bedtime could help increase skin temperature at the start of the night, and a timed electric blanket could be used to increase skin temperature in the morning. Thick blankets or an all-night electric blanket won’t help because they will simply cause overheating, especially of core body temperature, which will disrupt sleep.
“The effects of even very minimal temperature manipulations within the thermoneutral comfortable range are so pronounced that they warrant further research into practical thermal manipulation applications to improve sleep,” the researchers concluded.
Raymann, R.J., Swaab, D.F., Van Someren, E.J. (2008). Skin deep: enhanced sleep depth by cutaneous temperature manipulation. Brain, 131(2), 500-513. DOI: 10.1093/brain/awm315