Circadian temperature rhythm blunting and sleep composition

The sleep patterns of nine male subjects were studied on four consecutive nights comprising two baseline nights, one night on which environmental temperature was elevated from 21 degrees C to 30 degrees C one hour after lights out, and a recovery night. There was a suppression of stage 4 sleep during the initial three hours of sleep on the hot night. A significant increase in stage 4 sleep with a decrease in stage 2 sleep occurred during the first three hours of the recovery sleep. There was a shortening in sleep onset latency and an increase in sleep efficiency on the recovery night. There were no changes in REM latency or REM sleep time. Rectal temperature rose after the increase in ambient temperature on the hot night. These results indicate that elevations in environmental temperature during sleep affect sleep patterns in a manner opposite to elevations of body temperature occurring prior to sleep onset. The curtailing of the usual circadian temperature drop during the first few hours of sleep reduces slow-wave sleep during this period. These findings have implications for those conditions with both altered sleep and altered temperature rhythms, for example, depression.