Competition Phase Modulates Diurnal Variation in Olympic Swimming Performance (2004-2024)

To examine the association between time of day and performance across competition phases in Olympic swimming, considering scheduling structure and event distance.

A total of 3510 performances from 705 swimmers across 6 Olympic Games (Athens 2004-Paris 2024) were analyzed using linear mixed-effects models. Time of day was entered as a continuous predictor, with competition phase (heats, semifinals, and finals), stroke, sex, Olympic year, and event distance as covariates, and swimmer identity as a random intercept. Interaction terms between time of day and competition phase were tested, and supplementary analyses compared Olympic editions with morning finals (Beijing 2008 and Tokyo 2020) versus evening finals.

Time of day was significantly associated with standardized performance overall (β = -0.05, P < .001). A significant time-of-day × competition phase interaction showed that diurnal effects differed across rounds, being most pronounced during heats (β = -0.055), attenuated during semifinals, and showing only a small positive trend in finals not confirmed in the finals only analysis. A finals-only model confirmed no significant association between time of day and standardized performance (β = 0.04, P = .456). Olympic editions with morning-final schedules were associated with faster standardized performances than editions with evening-final schedules (β = -0.27, P < .001), although this should be interpreted as an effect of competition scheduling and circadian adaptation rather than an intrinsic morning-performance advantage. Event distance did not consistently moderate diurnal patterns.

Time-of-day effects in Olympic swimming are strongly modulated by competition phase and scheduling structure, being most detectable during heats and attenuated or masked in finals.