Acute finger temperature changes preceding sleep onsets over a 45-h period

A resurgence in the field of sleep initiation and thermoregulation has seen a number of investigators reporting relatively gradual increases in distal skin temperatures of the hands and feet prior to sleep onset at typical bedtimes. The present study extends upon prior knowledge by investigating whether: (1) this is a change of distal skin temperature triggered specifically by the attempt to fall asleep and (2) whether this relationship holds for various phases of the circadian rhythm whenever sleep is attempted. Fourteen healthy good sleepers participated in a modified 45-h constant routine (CR) with multiple sleep onset latency tests (MSLT) conducted every half hour. After a brief decrease of finger temperature associated with small postural adjustments at the beginning of each sleep latency test, finger temperature showed rapid (0.8 degrees C min-1) increases of 1-3 degrees C leading up to the onset of sleep. This rapid increase of finger temperature was relatively consistent across the 45-h CR, despite very significant circadian variation of the pre-MSLT baseline finger temperature and homeostatic decrease of sleep latency.