Core temperature and sweating onset in humans acclimated to heat given at a fixed daily time.
Level 4 - case-series / case-control
Uncontrolled pre-post physiological laboratory study in a small group of human volunteers.
PubMed 10198390 · doi:10.1152/ajpregu.1999.276.4.R1095
What was done
Human volunteers completed a heat acclimation protocol consisting of daily exposure to 46°C ambient temperature and 20% relative humidity for 4 hours (1400–1800) over 9–10 consecutive days. In the first experiment, 24-hour rectal temperature was measured at 27°C ambient temperature in six subjects before and after heat acclimation. In the second experiment, six subjects underwent a 30-minute leg-immersion thermal challenge (42°C water at 28°C ambient temperature, 40% relative humidity) in the morning (0900–1100) and afternoon (1500–1700) before and after acclimation, assessing rectal temperature and sweating responses at the chest and forearm.
What was found
The abstract does not report exact numerical values or effect sizes. Rectal temperature over 24 hours was significantly lower after acclimation exclusively between 1400 and 1800. In the thermal challenge experiment, heat acclimation significantly shortened sweating latency and lowered the rectal temperature threshold for sweating in the afternoon test window (1500–1700), with no significant changes in the morning test window (0900–1100).
Why it matters
The findings suggest that human autonomic thermoregulatory adaptations to repeated heat exposure can be circadian- or time-specific, aligning with the specific time of day of prior heat exposure.
Limits
The sample size is very small (n = 6), and the abstract does not specify whether the same individuals participated in both experiments. There was no parallel control group to rule out time-dependent or seasonal confounders, no numerical data or confidence intervals are provided in the abstract, and persistence of the acclimation was not tested.
Cited by
- supports Repeated heat exposure causes heat acclimation, prompting the body to begin sweating at a lower core body temperature threshold.