Sleep is the primary recovery mechanism for athletes. During slow-wave sleep, growth hormone peaks. During REM sleep, motor patterns are consolidated.
The effect size is large: restricting sleep to 6 hours/night for two weeks produces cognitive impairment equivalent to 24 hours of total sleep deprivation — and subjects don't perceive themselves as impaired.
For athletic performance: reaction time, accuracy, sprint speed, and endurance all decline measurably with sleep restriction. No recovery modality (ice baths, massage, nutrition) compensates for inadequate sleep.
See: progressive-overload