SummaryAlterations in thermoregulatory mechanisms related to sleep state may play an important role in the problems of homeostasis experienced by the premature infant. In the adult, homeothermic regulation of body temperature may be suspended during REM. We measured oxygen consumption (&OV0312;O2) in six premature infants 33–35 wk gestation both at thermoneutrality and during a mild thermal stress to determine whether thermoregulatory responses were intact during REM sleep. All infants were studied under radiant warmers. Skin temperature was allowed to fall 7–8 times during a 6–8 h study period while &OV0312;O2, &OV0312;CO2, heart rate and TcPO2were continuously recorded. Sleep state was scored using EEG, EOG, EMG and behavioral criteria. A total of 1,162 one-min epochs were scored. In all states including REM, &OV0312;O2was significantly higher during the cool periods. The mean increases: 21.5%, 23.3%, 11.1% and 5.3% for Awake, Indeterminate, REM and NREM respectively. When REM and NREM were compared at thermoneutrality, there was no difference in the &OV0312;O2(8.80 ± 0.11 and 8.93 ± 0.15 cc/kg/min, mean ± S.E., for REM and NREM, respectively). We conclude that in contrast to the adult, active thermoregulation occurs in the premature infant during REM sleep.SpeculationIn the adult, the reason for temporary poikilothermy during REM sleep is unknown. Some have considered this phenomenon a regression to a phylogenetically primitive condition which is necessary for normal hypothalamic function. Because the newborn period is characterized by profound ontogenic change, it is reasonable to speculate that the difference we observed between the premature and the adult is due to a developmental phenomenon. We further suggest that this developmental process may serve to protect the premature infant during REM sleep from extended periods of poikilothermy.