The Influence of Time of Day on Critical Swimming Speed in Chronotype-Screened (Neither-Type) Well-Trained Male Swimmers

Background: Critical swimming speed (CSS) is widely used to prescribe training and monitor endurance-related change, yet it is unclear whether its estimate varies with time of day when chronotype and relevant pre-trial conditions are considered. Purpose: To compare CSS at 09:00 h and 17:00 h in well-trained male swimmers with a homogeneous neither/intermediate (N-type) chronotype. Methods: Sixteen well-trained male front-crawl swimmers (age 17 ± 1 yr; height 1.78 ± 0.03 m; body mass 69.18 ± 4.56 kg; experience 11 ± 2 yr; weekly swimming volume ≈45 km) completed morning and evening sessions in a randomised, counter-balanced order, separated by 48 h. Each session comprised a 50 m trial followed by a 400 m trial, from which CSS was calculated. Twenty-four-hour energy and carbohydrate intake, forehead skin temperature, resting and post-trial heart rate, the Hooper Index, and Borg CR-10 RPE were also assessed. Results: CSS, the pre-specified primary outcome, was significantly higher in the evening than in the morning (morning, 1.46 ± 0.03; evening, 1.47 ± 0.05 m·s−1; Δ = +0.68%; p = 0.045; d_av = 0.24). The 400 m time was significantly faster in the evening (morning, 265.98 ± 6.25; evening, 264.37 ± 8.07 s; Δ = −0.61%; p = 0.047), whereas the 50 m time did not differ (p = 0.380). Forehead skin temperature and resting heart rate were higher in the evening (both p < 0.001); all other physiological, perceptual, well-being, and dietary comparisons were non-significant. Conclusions: A small evening increase in CSS, accompanied by faster 400 m performance, was observed in this homogeneous N-type cohort. The physiological measurements were concurrent observations and do not establish a mechanism. Standardising the time of day of repeated CSS assessment is recommended; larger, sex-balanced, and chronotype-diverse studies are needed before applying a numerical time-of-day correction.