Current thermal comfort theories and standards are mainly concerned with people in waking state. The effects of air temperature on sleep quality and thermal comfort of sleeping people were investigated in this study by experimenting on human subjects. Sleep quality was evaluated by subjective questionnaires performed in the morning as well as electroencephalogram (EEG) signals, which were continuously recorded during the all-night sleep period. Subjective assessments on thermal comfort were performed both before and after sleep. Analysis on EEG signals indicated that the subjects took longer time to fall asleep and experienced shorter period of slow wave sleep (SWS) when the room temperatures moderately deviated from neutral.