In recent years, tropical nights and deaths caused by heatstroke have increased in summer. Air conditioner use is recommended during sleep, but there is no clear statement on how to use it. Results of earlier studies have indicated that sleeping with windows open improves sleep quality. After measuring the temperature and humidity and conducting questionnaire surveys about summer sleep, we compared sleeping practices with the windows open and with cooling all night, particularly emphasizing fatigue recovery. Choice of natural ventilation or air conditioning is related to residents and residence attributes, but unrelated to mean outdoor temperatures during sleep in peak summer. Fatigue recovery scores were found to be related to mean bedroom temperature during sleep using air conditioning. However, no relation was found for natural ventilation nights. For fatigue recovery, it is better to sleep with windows open when the outside temperatures are lower than 28 ℃, and to set the air conditioner to 29 ℃ or 30 ℃ when the outside temperatures are higher than 28 ℃.