The relationship between sleep and thermoregulation has been highlighted by numerous studies conducted on both humans and different animals. Before illustrating the results of this research, a brief introduction on sleep is needed, which as many will know is not the same throughout its duration but consists of two main phases, the non-REM (orthodox sleep) and the REM ( paradoxical sleep). The analysis of the term REM allows us to appreciate the substantial differences between these two phases; REM is in fact an acronym of rapid eye movements = fast eye movements .
Conversely, non-REM sleep is characterized by the drowsiness of all autonomic functions, as evidenced by an EEG trace characterized by large and slow waves.
The phases of REM sleep, interspersed with longer periods of deep sleep, last 15-20 minutes each and are repeated approximately every two hours. During the night the phases of orthodox sleep progressively diminish and those of REM type increase in duration and intensity, up to the longest moment before waking up.
According to what has been said, it is not surprising that the appearance of non-REM sleep is accompanied by a reduction in metabolic processes, evidenced by the decrease in cardiac activity and central body temperature, and by the increase in cutaneous activity. During non-REM sleep, therefore, thermoregulation is oriented towards a reduction in body temperature itself. Furthermore, it seems that the generalized decline in metabolic activity is mediated by a thermoregulatory control that anticipates the appearance of sleep; therefore, the reduction of metabolic processes seems to be not only the consequence, but also a necessary requirement to favor sleep. During the non-REM sleep phase the body maintains its thermoregulatory capacity; consequently, similarly to what occurs in the waking state, it responds to an increase in environmental temperature with polypnea, peripheral vasodilatation, sweating, reduction of the metabolic processes underlying optional thermogenesis and assumption of a posture favoring thermal dispersion; vice versa, when the ambient temperature decreases, during non-REM sleep it is possible to appreciate cutaneous vasoconstriction, piloerection, increase in optional thermogenesis and, limited to phases one and two, appearance of the shiver. What has been described does not occur in the phases of REM sleep, during which the hypothalamus loses the ability to thermoregulate, to the point of allowing a paradoxical behavior to be appreciated with the appearance of cutaneous vasoconstriction in animals exposed to heat and vasodilation in those exposed to cold. Consequently, during REM sleep the only solutions to safeguard the body from excessive temperature changes are represented by the awakening or the passage to a phase of non-REM sleep. Cerri et al., 2005, have shown for example how the exposure of rats to cold environmental conditions, such as to lead to the loss of thermoneutrality, has produced a reduction of about 80% in the duration of REM sleep normally expressed in this species .
