The effects of distal limb warming on sleep latency.

Sleep onset is best predicted by an increase in the amount of hand and foot warming relative to more proximal areas (Krauchi, Cajochen, Werth, & Wirz-Justice, 1999; Krauchi, Cajochen,Werth,& Wirz-Justice, 2000).We assessed if an increase in sleepiness would also occur if the distal limbs were warmed through an external manipulation. In this study, 5 min before participants (N = 11) attempted to fall asleep on a multiple sleep latency test (Carskadon et al., 1986) their hands and feet were immersed in water heated to either 42 degrees C or heated to the temperature of the warmest limbs. The results show no difference in sleep latency between the warm and control water conditions. There was a decrease in sleep latency in the control and warm water conditions compared to the initial (noncounterbalanced) baseline multiple sleep latency test. Further studies are necessary to determine if any degree of limb warming hastens sleep onset.