Researchers writing in the medical journal Physiology and Behavior say they "present new data indicating age- and insomnia-related changes in the sleep-onset latency response to foot warming, and evaluate whether different methods of foot warming could provide an applicable strategy to address sleep complaints."

Physiol Behav. 2007 Feb 28;90(2-3):257-66.
Raymann RJ, Swaab DF, Van Someren EJ. Skin temperature and sleep-onset latency: Changes with age and insomnia.

Throughout the 24-hour day, the occurrence of sleep and wakefulness is closely related to changes in body temperatures.

Changes in skin temperature may causally affect the ability to initiate and maintain sleep.

First, we briefly summarize a previously proposed neurobiological mechanism that couples skin temperature to sleep propensity. Next we review previous findings on the relation between skin temperature and sleep-onset latency, indicating that sleep propensity can be enhanced by warming the skin to the level that normally occurs prior to - and during - sleep. Finally, we present new data indicating age- and insomnia-related changes in the sleep-onset latency response to foot warming, and evaluate whether different methods of foot warming could provide an applicable strategy to address sleep complaints. Foot temperature manipulations included footbaths before sleep onset (1), and heatable bed socks applied either before (2) or after lights-off (3).

In adults, sleep-onset was accelerated by warm and neutral bed socks after lights-off and correlated to the increase in foot temperature. This increase was attenuated in elderly subjects. In elderly subjects without sleep difficulties, sleep onset could be accelerated with neutral bed socks after lights-off and a warm footbath prior to lights-off. In elderly insomniacs, none of the treatments accelerated sleep onset. We illustrate that elderly subjects show an attenuated increase in foot temperature after lights-off and lose the relationship between pre-sleep heat-loss activation and sleep latency. The sensitivity of sleep propensity to foot warming changes with age and is attenuated in age-related insomnia.