Higher evening metabolic responses contribute to diurnal variation of self-paced cycling performance

This study examined the effect of time of day (TOD) on physical performance, and physiological and perceptual responses to a 10-km cycling time trial (TT ). Twelve physically trained subjects (20.3 ± 1.2 years, 74.3 ± 7.4 kg, 179.7 ± 5.5 cm) completed, in a randomized order, a TT in the morning and in the evening. Intra-aural temperature (IAT) was measured at rest and following the TT . Completion time, power output (PO), rating of perceived exertion (RPE), heart rate (HR), minute ventilation (V̇E), oxygen uptake (V̇O ), carbon dioxide production (V̇CO ) and respiratory exchange ratio (RER) were assessed every km during the TT . Blood lactate concentration [La] and blood glucose concentration [Glu] were assessed before, during and immediately after the TT . Faster completion time (Δ = 15.0s, p = 0.03) and higher IAT (Δ = 0.33°C, p = 0.02 for pre-TT ) were obtained in the evening compared to the morning with a significant correlation between Δ completion time and Δ IAT at post-TT (r = -0.83, p = 0.04). V̇O , [La] and [Glu] increased significantly during both test sessions (p < 0.001) with higher values in the evening compared to the morning (p = 0.015, p = 0.04, p = 0.01, respectively). However, the remaining parameters were found to be only affected by the TT (p < 0.001). The TT generates a higher V̇O and higher [La] and [Glu] responses, contributing to a better cycling performance in the evening compared to the morning. The similar magnitude of the TOD effect on completion time and IAT at post-TT confirms that core temperature is one of the underlying factors contributing to the diurnal variation in physical performance.