Core temperature (Tc) shows rising (05:00-17:00 hours) and falling (17:00-05:00 hours) phases.

A new study published in the European Journal of Applied Physiology examined the time-of-day effects on endurance exercise capacity and heat-loss responses to control Tc in heat, at around the midpoint of rising and falling phases of Tc.

Here, 10 males completed cycling exercise at 70% peak oxygen uptake until exhaustion, in hot temperature—30 °C, 50% relative humidity. Participants commenced exercise in the late morning at 10:00 h (AM) or evening at 21:00 h (PM).

The findings revealed that the time to exhaustion was 28 ± 13% (mean ± SD) longer in PM (49.1 ± 16.3 min) than AM (38.7 ± 14.6 min). Moreover, Tc before and during exercise were higher in PM than AM, in accordance to the diurnal variation of Tc. On the other hand, rates of rise in Tc, mean skin temperature, thermal sensation and rating of perceived exertion during exercise were slower in PM than AM. However, dry and evaporative heat losses and skin blood flow during exercise were greater in PM than AM. Furthermore, during the 30-min post-exercise recovery, the rates of fall in Tc and skin blood flow were faster and thermal sensation was lower in PM than AM.

The results indicated that endurance exercise capacity is greater, and heat-loss responses to control Tc during and following exercise in heat are more effective, in the late evening than during mornings. While the perceived fatigue during exercise and thermal perception during and following exercise are lower in the late evening than morning.

Source: European Journal of Applied Physiology. 2020 Feb;120(2):481-491. doi: 10.1007/s00421-019-04292-6.