Toe temperature change: a measure of sleep onset?

In an attempt to provide a more "physiological" measure of sleep onset, temperature of the great toe was monitored for 166 sleeper-nights in both insomniac and control groups. All subjects were women between 21 and 45 years of age. Earlier published reports had suggested that toe temperature elevation was the result of lowered sympathetic tone and marked a "vegetative preparedness for sleep." More recently, elevation of the toe temperature has been used to denote a level of chemical anesthesia sufficient for major surgery. In this study, however, the mean latency of the time course of the temperature elevation of the toe was uncorrelated with sleep onset determined by EEG criteria. Temperature elevations were frequently of large magnitude but were not uniformly observed across all subjects and all nights. It is suggested that the toe temperature elevation is reflective of metabolic adjustments which coincide with the onset of the night's first period of slow wave sleep.