Effects of a moderate nocturnal cold stress on daytime sleep in humans

The effects of a nocturnal exposure to a cool environment on daytime recovery sleep was studied in eight young (20-25 years old) healthy volunteers. A set of standardized clothing (KSU ensemble type) was provided to each individual (estimated total thermal resistance: 0.6 clo). The subject kept awake was passively exposed from 22.30 to 07.30 hours to environments perceived as neutral (N) and comfortable or slightly cold (C) and uncomfortable. They were then allowed to sleep ad libitum (light out at 08.00 hours) under thermoneutral conditions (air temperature: 21 degrees C to 22 degrees C; clothing: cotton tee-shirt and pajama-pants; covering: one cotton sheet and one wool blanket). Sleep was recorded and scored according to the Rechtchaffen and Kales standard procedures. Esophageal temperature (T(es)) was recorded from 21.30 hours until the end of sleep. The nocturnal drops in T(es) were significantly different between N and C (p<0.01), this difference disappearing during sleep. No statistical difference was found between conditions for most of the sleep variables. Compared to N however, C resulted in a significant increase in rapid eye movement (REM) sleep duration (+35%, p<0.01) during the subsequent daytime sleep. It is hypothesized that the REM-sleep increase induced by the exposure to moderate cold is due to the thermal discomfort stress consciously perceived by the subject.