Thermoregulatory responses to cold water at different times of day

This study examined how time of day affects thermoregulation during cold-water immersion (CWI). It was hypothesized that the shivering and vasoconstrictor responses to CWI would differ at 0700 vs. 1500 because of lower initial core temperatures (T-core) at 0700. Nine men were immersed (20 degrees C, 2 h) at 0700 and 1500 on 2 days. No differences (P > 0.05) between times were observed for metabolic heat production ((M) over dot, 150 W.m(-2)), heat flow (250 W.m(-2)), mean skin temperature ((T) over bar T-sk, 21 degrees C), and the mean body temperature-change in (M) over dot (Delta(M) over dot) relationship. Rectal temperature (T,,) was higher (P < 0.05) before (Delta = 0.4 degrees C) and throughout CWI during 1500. The change in T,, was greater (P < 0.05) at 1500 ( - 1.4 degrees C) vs. 0700 (-1.2 degrees C), likely because of the higher T-re-(T) over bar(sk) gradient (0.3 degrees C) at 1500. These data indicate that shivering and vasoconstriction are not affected by time of day. These observations raise the possibility that CWI may increase the risk of hypothermia in the early morning because of a lower initial T-core.