Effects of time of day on self-paced performances of prolonged exercise

Pre-exercise body temperature varies throughout the day due to its circadian rhythm. This study aimed to compared responses to sustained exercise in the morning and in evening. Rectal temperature was measured pre-exercise and throughout a 60 min test on a cycle ergometer against a fixed frictional resistance in seven males (aged 19-24 years). The subjects were instructed to work as hard as possible over the entire exercise period but could vary the pedal frequency at any time. Power output was calculated by computer, utilising an optical detection device to monitor flywheel revolutions. Tests were performed at 08:30 and 17:30 h, balanced for order with at least 72 hours between tests. Rectal temperature was lower pre-exercise in the morning (37.2 degrees C) than at evening (37.8 degrees C). This differences was reduced to 0.3 degrees C by the end of exercise. Mean power output was similar for the two times of day for the whole exercise period. A higher power output in the evening over the first half of the test was compensated for by a greater performance in the morning over the second 30 min (p < 0.05). It seems that the pacing of endurance performance is affected in the morning, but without any overall effect at least for the duration examined and at the ambient environmental temperature of 17-19 degrees C.