Investigation of diurnal variation in sustained exercise performance

Human performance generally varies in phase with the circadian curve in body temperature. This relationship between performance and core body temperature may be disrupted when exercise causes a pronounced rise in body temperature. In this study the authors compared responses to exercise in the morning and in the evening when pre-exercise body temperature differed significantly. Rectal temperature was measured pre-exercise and throughout a cycle ergometer test at 70% VO2 max in seven males (aged 19-24 years). The test was performed at 08:30 and 17:30 h, balanced for order with 3 days minimum between tests. Onset of sweating, weight loss and time to exhaustion were recorded. Metabolic measures (VO2, VE and RER) and heart rate were recorded pre-exercise and during exercise. Rectal temperature and skin temperature at three sites were also measured. Rectal temperature and heart rate were lower in the morning than in the evening by 0.6 degree C and 7 beats min-1 (p < 0.01), respectively. The lower body temperature in the morning persisted throughout exercise, the final rectal temperature being 38.6 (SD = 0.2) degrees C in the morning and 39.2 (SD = 0.5) degrees C in the evening. No effects of time of day were observed for VO2, VE, and RER but heart rate values during exercise were lower in the morning for 30 min (p < 0.05). Sweat onset occurred sooner at 08:30 h (7.57, SD = 0.90 min) compared to 17:30 h (8.71, SD = 2.20 min) (p < 0.05). Time to exhaustion ranged from 48 to 72 min, but did not vary with time of day (p > 0.05). It is concluded that continuous submaximal exercise is not necessarily disadvantaged by a morning timing.