Nocturnal lowering of thresholds for sweating and vasodilation

Six subjects exercised on a bicycle ergometer at 60-70% of maximal aerobic power in a 25 degrees C ambient. Experiments on each subject were conducted at night (4:00-5:30 A.M.) and in daytime (noon-4:30 P.M.). Chest sweating rate (msw) was measured with resistance hygrometry. Forearm blood flow (BF), with an arm skin temperature of 35.5 +/- 1.2 degrees C (SD), was measured with electrocapacitance plethysmography. Esophageal temperature (Tes) was measured with a thermocouple at the level of the left atrium, and mean skin temperature (Tsk) was calculated from a weighted average of temperatures at three sites. Tes was corrected to a skin temperature of 33 degrees C as follows: T'es = Tes + (Tsk - 33 degrees C)/8. This correction reflects the relative contributions of Tes and Tsk to control of msw:T'es and BF:T'es relations were not consistently changed. In any given subject, thresholds for sweating and vasodilation were shifted about equally. These shifts averaged 0.57 degrees C (range: 0.23-0.93 degrees C)for msw and 0.63 degrees C (range: 0.17-0.98 degrees C) for BF.