My grandmother used to assure me that there was nothing like warm feet to ensure sound sleep. She told me tales of how, in rural Suffolk, she would warm a domestic brick in the oven, wrap it in an old blanket and push it to the bottom of the bed before the children took their place there. Later came the heavy stone bed-warmer, filled with hot water, again wrapped in a blanket. Today stone hot-water bottles are collectors' pieces.
After my grandmother's time came the corrugated aluminium warmer, and even later the rubber or plastic bottle which had its own fancy jacket. The electric blanket, in my tender youth, was rather frowned upon, and never appealed to me like the old stone bottles.
There is nothing new under the sun, and in Nature for September 2 a group based at a Basel psychiatric clinic has published the results of a study of the effects of distal vasodilatation on the onset of sleep in 18 healthy young men. Conditions of lighting and ambient temperature, and eating, were standardised, while core and distal temperatures were recorded. The greater the vasodilatation in hands and feet, indicated by limb temperature, the shorter was the time taken for the subject to fall asleep.
Vasodilatation, which occurs in early evening with the increased degree of sleepiness, is governed by the circadian clock controlled by the hypothalamus. The heat of the body is redistributed from the core to the hands and feet. Hypnotics and other aids to promote sleep act by dilating distal blood vessels and promoting heat transfer from the body core. This vasodilatation is also affected by the secretion of melatonin at the critical period of the day. Thus, a hot-water bottle applied to the feet, although it does not directly affect the central nervous mechanism regulating the sleep cycle, promotes the onset of sleep by encouraging heat transfer from the body core to the periphery.