Is there a diurnal variation in 4-km cycling time-trial performance, conducted in an environmental chamber where a standardised approach has been employed?

We investigated the hypothesis that a daily-variation exists for a 4-km time-trial, where a standardised-approach has been employed. Twenty-six male students completed a 30-s Wingate test, a VO2 peak test: and then three 4-km familiarisations at 12:00 h. Two further time-trials counterbalanced for start at 07:30 or 17:30 h were undertaken. There was 72 h between each session. Heart rate, rectal and skin temperature values, rating-of-perceived-exertion, effort, pacing perception and split-times were measured every 1-km during the time-trial. Blood lactate, urine osmolarity, rectal and skin temperature were taken at rest and immediately after each time-trial. Evening finishing times, resting rectal temperatures, pre-post time-trial blood lactate were faster, higher with a greater rise (similar to 10 s, 0.48(degrees)C, 0.7mmol.L) than morning values (p<0.05) with no other interactions. In summary, where a standardised approach has been employed 4-km performance was 2.6 % faster in the evening than the morning (ES = 0.34, power = 0.8).