In the thermal manikin experiments, the clothing insulation values (IT) for different combinations of parameters (clothing, material of quilt, quilt usage pattern, and postures) were measured. Subsequently, a formula for estimating IT was formulated. Furthermore, body movements were observed and the thermal environment in bedrooms were measured in a field survey. By applying the IT to the field survey result, the comprehensive thermal environment during sleep was evaluated. Additionally, sleep stages were determined from the subjects' electroencephalogram, electromyogram, and eye movement. The relationship between thermal environment and sleep quality was evaluated. Result: Evaluation of the sleeping thermal environment with adequate accuracy was possible considering clothing change. Wake per unit time was short for SET * of about 30 oC, so it is possible that a higher sensible temperature is preferred during sleep.