Temperature and Sleep Core Body Temperature (tc) Has Been Consid- Ered a “gold-standard” Marker of Human Cir- Cadian Rhythm, and It Is Strongly Related To

CORE BODY TEMPERATURE (TC) HAS BEEN CONSIDERED A “GOLD-STANDARD” MARKER OF HUMAN CIRCADIAN RHYTHM, AND IT IS STRONGLY RELATED TO sleep regulation. The human sleep-wake cycle is generally synchronized with the circadian rhythm of Tc, partly because the propensity for sleep reaches its maximum during the declining phase of Tc and partly because arousal is promoted after Tc has passed its nadir.1,2 Tc decreases rapidly about 60 minutes prior to the onset of sleep,3 and it decreases further during slow-wave (stages 3+4) sleep (SWS), due to a combination of decreases in metabolism, vasodilation, and an increase in sweating.4,5 Sleep structure is also related to changes in Tc. SWS is decreased if the usual decrease of Tc is restricted by an ambient temperature (Ta) above the thermoneutral zone 6 (here defined as the range of Ta over which temperature regulation is achieved by control of sensible heat loss, without regulatory changes in either metabolic heat production or evaporative heat loss7). The thermoneutral zone itself shows a circadian variation, reflecting a corresponding circadian rhythm in the body temperature set point.8 In contrast, stage 4 sleep is increased when the normal nocturnal decrease of Tc is augmented by a constant, mild cold stress, although sleep efficiency (the percentage of time asleep relative to the time spent in bed) is decreased.9 Thus, during the customary hours of sleep Ta is an important determinant of both the quality and the quantity of sleep achieved,10 and the appearance of SWS seems to be facilitated by a greater nocturnal decrease of Tc. Gradual changes of Ta within the thermoneutral range while a person is sleeping can affect Tc and sleep structure without inducing arousal.11 In the experiments of Dewasmes and associates,11 Ta was maintained at 29 oC for 60 minutes after lights-out; it was then decreased linearly from 29oC to 26oC over the period 60 to 180 minutes, increased from 26oC to 32oC between 180 and 420 minutes, and decreased back to the initial value of 29oC from 420 to 480 minutes after lights-out. This pattern of change advanced the nadir of Tc (rectal temperature, Tre) and also increased the amplitude of change in Tc during nocturnal sleep. Dewasmes et al 11 noted further that, under these conditions, the peak propensity for rapid eye movement (REM) sleep was advanced and the amount of REM sleep tended to increase. Based on these observations, we hypothesized that, if, in contrast, the nadir of Ta was delayed by a slower change in Ta within the thermoneutral zone, then Tc would reach a lower value after a longer delay and the amount of SWS would be increased, without any loss of sleep efficiency. We decided to test this hypothesis, in order to increase our understanding of how externally induced changes in Tc could enhance sleep structure.