In this study, the method to evaluate thermal environment during sleep was proposed, with the aim of improvement of evaluation accuracy of thermal environment considering thermal insulation of bedding system and thermal conductance by changing sleep posture. To measure thermal insulation of bedding system and thermal conductance of contact surface with bedding, the experiment using thermal manikin was conducted in climate chamber in 2016. Thermal insulations of bedding system under 20 conditions of different bedding hanging type and sleeping posture were measured. Also, to examine thermal environment evaluation method, a field survey was conducted in July, August and September 2017 in actual bedrooms, and used polysomnography to evaluate sleep quality. The thermal environment and physiological parameters were continuously monitored during sleep in each subject's bedroom. To evaluate temporal change of sleeping environment, sleeping appearance was measured by video. The subjects were 18 young and 5 middle aged persons who did not possess any sleep disorders. Each subject's sleep was monitored for five nights.<br> It was confirmed that the relationship between the ratio of the body surface area of the part where the body and the bedding contact and thermal insulation is exponential. Compared with the supine position, thermal insulation in the lateral decubitus position was lower, and thermal insulation was changed also depending on ratio covered by futon. Based on the measured value by experiment, the presumption formula witch predicts thermal insulation considering clothes, quilts, comforters, sleeping postures was proposed. Also, it was showed that the heat flux between the mattress and the back in the supine position decreased exponentially as time elapsed. By converting the measured data to the heat load of whole body and considering the influence of the ambient temperature, the inference formula of heat conduction caused by the change of the contact surface with the bedding by turning over was proposed.<br> Proposed estimated equation of thermal insulation and heat conduction by turning at sleep was applied to video data measured by field survey in actual bedrooms. As a result of evaluating the thermal environment considering the change in thermal insulation and thermal conduction, it was found that the relationship between heat load of human body and wake after sleep onset(WASO) was stronger than the case of assumption that the environment does not change throughout the night. It was confirmed that the evaluation accuracy of the thermal environment improved by appropriately evaluation of the change of thermal environment over time during sleep. The usefulness of appropriate thermal environment evaluation by using this proposed method was shown.<br> As a result of analyzing heat load of human body which was calculated by the proposed method, it was confirmed that WASO without turning occurred and the ratio of WASO increased in an environment away from thermal neutrality. Also, it was revealed that turning without WASO occurred, and at the time of thermal neutrality, the ratio of WASO was about 50%, not necessarily happen WASO. In both cases, the ratio of WASO increased in an environment deviating from thermal neutrality. And, relationship between human heat load and ratio of WASO showed that the ratio of WASO was significantly increased when away from thermal neutrality. The width of the thermal acceptance zone during sleep was wider than at arousal time, indicating the possibility of accepting a slightly warm environment.