CIRCADIAN CHANGES IN THE SWEATING-TO-VASOCONSTRICTION INTERTHRESHOLD RANGE

Thermoregulatory defenses are characterized by thresholds, the core te mperatures triggering each response. Core body temperature is normally maintained within the interthreshold range, temperatures between the sweating and vasoconstriction thresholds that do not trigger autonomic defenses. This range usually spans only some 0.2 degrees C, but it re mains unknown whether similar precision is maintained during the circa dian core temperature cycle of about 0.8 degrees C. Accordingly, we ev aluated the interthreshold range at four times of the day. We studied ten male volunteers, each at 3 a.m., 8 a.m., 3 p.m.. and 8 p.m. At lea st 12 h elapsed between tests, and the order was randomly assigned. At each study time, volunteers were warmed peripherally until sweating w as observed. Skin temperature was subsequently kept constant while cor e temperature was decreased by central-venous infusion of ice-cold flu id until peripheral vasoconstriction was detected. The volunteers were not permitted to sleep during threshold determinations, although slee p was not otherwise controlled. The core temperature triggering an eva porative water loss of 40 g.m(-2).h(-1) identified the su eating thres hold. Similarly, the vasoconstriction threshold was defined by the cor e temperature triggering the initial decreases in plethysmographic fin ger tip blood flow. The interthreshold range at 3 a.m. was tu ice that observed at the other study times (P < 0.05). Our data suggest that a utonomic control of body temperature is reduced at 3 a.m.. even when s leep is denied. This result contradicts the general perception that ci rcadian variation alters the thermoregulatory target temperature, but not precision of body temperature control.