Comfortably cool bedroom environment during the initial phase of the sleeping period delays the onset of sleep in summer

Air conditioners are becoming commonly accepted as necessary and conventional in bedroom, but there are very limited researches on thermal comfort of sleeping people. The effects of three air temperature changes were investigated: a constant temperature reference condition (26 °C), a Fall-Rise change condition (28 °C-27 °C-26 °C-27 °C-28 °C), and a Rise-Fall change condition (25 °C-26 °C-27 °C-28 °C-27 °C-26 °C). Eighteen healthy young people (9 males and 9 females, mean age 23 years) slept in climate chamber and used typical summer covering in the three conditions, with continuous monitoring of multiple physiological parameters. Sleep quality was evaluated subjectively using questionnaires in the morning and objectively by analysis of electroencephalogram (EEG), electrooculogram (EOG) and electromyogram (EMG) signals which were continuously monitored during the sleeping period. Compared with the other two conditions, when the air temperature was kept at a lower but comfortable 25 °C during the initial phase of the sleeping period (Rise-Fall), the subjects had colder hands and feet until 90 min after lights off and took longer time to fall asleep, and they reported poorer sleep quality in the morning. In the Fall-Rise condition, subjects entered rapid-eye movement sleep later without changing sleep quality or thermal comfort. This study indicates that a comfortably cool environment during the initial phase of the sleeping period delayed the onset of sleep.