The time of day differently influences fatigue and locomotor activity: Is body temperature a key factor?

Abstract The aim of this study was to verify the possible interactions between exercise capacity and spontaneous locomotor activity (SLA) during the oscillation of core body temperature (T b ) that occurs during the light/dark cycle. Wistar rats (n = 11) were kept at an animal facility under a light/dark cycle of 14/10 h at an ambient temperature of 23 °C and water and food ad libitum. Initially, in order to characterize the daily oscillation in SLA and T b of the rats, these parameters were continuously recorded for 24 h using an implantable telemetric sensor (G2 E-Mitter). The animals were randomly assigned to two progressive exercise test protocols until fatigue during the beginning of light and dark-phases. Fatigue was defined as the moment rats could not keep pace with the treadmill. We assessed the time to fatigue, workload and T b changes induced by exercise. Each test was separated by 3 days. Our results showed that exercise capacity and heat storage were higher during the light-phase ( p 0 .05). In contrast, we observed that both SLA and T b were higher during the dark-phase ( p p