Brain temperature (Tbr), vigilance state, and electroencephalograph slow-wave activity (EEG SWA, 1.0-4.0 Hz) were measured during hibernation and spontaneous arousals to euthermia in seven golden-mantled ground squirrels (Spermophilus lateralis). Animals were held at air temperatures (Ta) ranging from 6 to 21 degrees C. SWA was used as a measure of the intensity of non-rapid eye movement (NREM) sleep. Squirrels that had hibernated at high Ta had lower SWA in NREM sleep in the hours following arousal than when they hibernated at low Ta. SWA in NREM sleep during euthermia immediately following arousal was significantly correlated to minimum Tbr and SWA during hibernation. The duration of the preceding hibernation bout had no significant effect on SWA during euthermia. We hypothesize that the restorative process of sleep, reflected by SWA, is temperature sensitive and is compromised by the low temperatures in hibernation. The accumulation of a SWA debt during hibernation may be related to the temperature-dependent depression of SWA during hibernation.