Abstract

Circadian rhythms in cognitive and athletic performance have been demonstrated. However, to our knowledge, no study has directly compared the characteristics of these rhythms. PURPOSE: We aimed to compare the circadian rhythm of reaction time performance with the circadian rhythm of swim performance. METHODS: Twenty-five participants were assessed for 50-55 consecutive hr in the laboratory. Throughout the duration of the laboratory protocol, all participants adhered to a 3-hr "ultra-short" sleep/wake cycle, a known chronobiological technique consisting of repeated 3-hr cycles of 2 hr of wakefulness in dim light followed by 1 hr of sleep in darkness. Beginning with the initial wake period and every third wake period thereafter (i.e., every 9 hr), participants performed a 5-min psychomotor vigilance task (PVT), with median reaction time as the outcome measure. In the wake period subsequent to PVT assessment, participants performed a maximal-effort 200-m freestyle swim. Overall, each participant performed 6 PVT assessments and 6 swim trials at randomly assigned times across the 24-hr day. A cosine fit of intra-aural temperature, measured three times each wake period, was used to establish the body temperature minimum (Tmin). PVT and swim performances were z-transformed (based on individual performance), expressed relative to Tmin, and subsequently placed into 2-hr bins relative to Tmin. RESULTS: ANOVA revealed significant (p<0.001) circadian patterns in PVT and swim performance. PVT performance was best between 9 and 7 hr before the Tmin (~2100 hr), and was worst from 1 to 3 hr after Tmin (~0700 hr). In contrast, swim performance peaked between 5 and 7 hr before the Tmin (~2300 hr) and was worst between 1 hr before to 1 hr after the Tmin (~0500 hr). Cosinor analysis revealed an earlier acrophase (i.e., fitted time of peak performance) for PVT performance (1738 hr) relative to swim performance (1803 hr). CONCLUSIONS: Although PVT and swim performance both demonstrated circadian patterns, their acrophases differed slightly, with swim performance slightly phasedelayed in relation to PVT performance. This study was supported by a Gatorade Sports Science Institute Student Grant, NIH grant HL71560, and a VA VISN-7 Career Development Award.

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