Abstract
The thermal environment is one of the most important factors that can affect human sleep. The stereotypical effects of heat or cold exposure are increased wakefulness and decreased rapid eye movement sleep and slow wave sleep. These effects of the thermal environment on sleep stages are strongly linked to thermoregulation, which affects the mechanism regulating sleep. The effects on sleep stages also differ depending on the use of bedding and/or clothing. In semi-nude subjects, sleep stages are more affected by cold exposure than heat exposure. In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation. On the other hand, cold exposure does not affect sleep stages, though the use of beddings and clothing during sleep is critical in supporting thermoregulation and sleep in cold exposure. However, cold exposure affects cardiac autonomic response during sleep without affecting sleep stages and subjective sensations. These results indicate that the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.
Highlights
The thermal environment is one of the most important factors that can affect human sleep
In cold exposure, shivering during sleep is confined to stages 1 and 2 and is not observed in slow wave sleep (SWS) and rapid eye movement sleep (REM) [35], while the Tsk of the extremities is decreased during REM compared to that at control conditions [36]. These results indicate that REM and thermoregulation are mutually exclusive and partly explain the decrease in REM observed during heat or cold exposure, bearing in mind that REM is more sensitive to Ta than other sleep stages
The effect on SWS does not change after partial sleep deprivation where sleep pressure is increased [45]. These results suggest a strong effect of heat load on sleep stages, which is related to thermoregulation during sleep
Summary
Heat exposure affects SWS and REM, whereas cold exposure does not affect sleep stages. Considering that a Ta of 32°C with 80% relative humidity affects only SWS without affecting REM [46], heat affects SWS first, whereas REM may be well-preserved in real-life situations. KM has been involved in drafting the manuscript and revising it critically for important intellectual content. Both authors read and approved the final manuscript. Author details 1Kansei Fukushi Research Center, Tohoku Fukushi University, 1-149-6 Kunimigaoka Aoba Sendai, Miyagi, 981-0935, Japan. Author details 1Kansei Fukushi Research Center, Tohoku Fukushi University, 1-149-6 Kunimigaoka Aoba Sendai, Miyagi, 981-0935, Japan. 2Department of Early Childhood and Primary Education, Tohoku Fukushi University, 6-149-1 Kunimi, Aoba-ku, Sendai, Miyagi, 989-3201, Japan
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