A field survey was conducted during summer in the actual bedrooms using polysomnography to evaluate the effect of thermal environment on sleep quality whilst considering sleep stages. Subjective sleep quality was measured by questionnaires. Objective sleep quality was evaluated by an analysis of electroencephalogram (EEG), electrooculogram (EOG), and electromyogram (EMG) signals, which were continuously measured during sleep. Subjects consisted of 16 students. Results showed that the effect of the air temperature on awakening varied depending on sleep stages, and the correlation between them were statistically significant. Moreover, the effects of the thermal environment were stronger during light sleep than during deep sleep. Above the neutral temperature, increasing the new standard effective temperature (SET*) disturbs sleep. The relationship between SET* and sleep quality was stronger than that between sleep quality and each individual thermal factor. It was concluded that the correlation between each factors of the thermal environment influences sleep quality.