It's during rest, especially sleep, that the body mends and grows stronger. While sleeping, growth hormone is released. If the time spent snoozing is shortened, it takes longer to recover. This compromises consistency, which is the single most important aspect of training. Reducing sleep time also means that glycogen stores aren't fully replenished between workouts leading to decay in endurance performance over several days.
Damaged cells take longer to heal raising the risk of injury and illness.
If the training workload remains high despite decreased sleep time, overtraining becomes a real threat. Burnout is waiting just around the corner. A well-rested athlete looks forward to workouts, enjoys training, is powerful, has good endurance, and progressively grows stronger. A tired athlete realizes none of these benefits. Never underestimate your need for sleep.