Sleep, Fatigue, Stress, and Training Load as Predictors of Next-Day Reaction Time in a 60-Day Single-Subject Study

This 60-day single-subject observational study examined how sleep duration, sleep quality, fatigue, stress, and prior-day training load were associated with next-day reaction time in a single adolescent athlete. Daily sleep, fatigue, and stress were self-recorded, training load was calculated from training duration and intensity, and five reaction-time trials were averaged into a daily reaction-time score. Across the study period, reaction time varied meaningfully from day to day. Fatigue showed the strongest association with reaction time, while sleep-related variables and stress also showed meaningful relationships. Training load was weaker and less consistent. Outlier-free comparisons showed a broadly similar pattern, although the outlier-free regression model was not statistically significant overall. Overall, the data suggest that next-day reaction time was more closely associated with recovery-related variables than with training load alone.