Evidence of a circadian variation in 10-km laboratory running time-trial performance, where a standardised approach has been employed

Diurnal variations in time-trial performance have been shown in people living normally, where a "standardised protocol" has been employed to reduce bias. We tested the hypothesis that a circadian variation exists for a 10-km running laboratory-based time-trial, where such a standardised approach is used. Twelve recreationally active adult males were recruited. The participants completed three familiarisation time-trials to the best of their ability at a self-selected pace and six 10-km time-trials at 06:00, 10:00, 14:00, 18:00, 22:00 and 02:00 h. Each session was separated by 7-days. Participants were allocated into 6 groups due to finish times (FT); sessions were counterbalanced in order of administration. A cosine fit for resting intra-aural temperature and FT both showed a significant circadian rhythm (<i>p</i> < 0.05) with mesor, amplitude and acrophases of 36.61°C vs 2994 s, 0.34°C vs 149 s; and 17:29 vs 18:44 h:min, respectively. The parallelism of temperature and FT agrees with previously published research. The finding of a 24-h rhythm in 10-km FT (5.0%, d = 0.80; power = 100%) concurs with that of a shorter distance where "standardised protocols" have been employed (4-km, 2.6%, d = 0.34). This finding has implications for scheduling of competition and training. Whether this variation is apparent in other populations, however, is unclear.