It is understood by many lifters that sleep is when your muscles rebuild. They will rebuild when you are awake but it is when you sleep that the magic happens. It's not just muscle repair that occurs when you sleep. Those strength gains you see in the gym are not tied solely to muscle repair. It is your deep NREM sleep that potentiates practiced motor skill consolidation. Those repeated squats and rows take practice and your brain takes consolidation for that practice to matter. Without consolidation, that practiced motor skill cannot cement itself in your brain, hindering your fitness goals and putting yourself at risk for injury.

In the years since our discovery, numerous studies have shown that sleep improves the motor skills of junior, amateaur and elite athletes across sports as diverse as tennis, basketball, football, soccer, and rowing. So much so that, in 2015, the International Olympic Committee published a consensus statement highlighting the critical importance of, and essential need for sleep in athletic development across all sports for men and women (Bergeron, et al, 2015).

Matthew Walker from his book Why We Sleep.
Several hormonal and neurological changes occur during puberty, affecting the homoeostatic and circadian regulation of sleep. Adolescents have later bedtimes and wake times, with a recommended optimal level of sleep of 8.59.5 h. However, international trends indicate a prevalence of insufficient sleep among adolescents, often prompted by early school-start times, academic demands, social activities and events, caffeine consumption and screen-time at night. For youth athletes, the training and competition schedule further exacerbates the deficiency of sleep, with preliminary evidence indicating an increased likelihood of injury with insufficient sleep. Given the potential consequences of insufficient sleep on health, behaviour, attention, and learning and athletic performance, interventions (eg, tailored training and even perhaps schooling schedules) to support adequate sleep in youth athletes should be implemented.

Reference:

Bergeron, M., Mountjoy, M., Armstrong, N., Chia, M, et al. (2015). International olympic committee consensus statement on youth athletic development. British Journal of Sports Medicine, 49(13), 843-51.
Described in a research study of competitive young athletes in 2014, you can see that a chronic lack of sleep across the season predicited a massively higher risk of injury.



Reference:

Milewski, M. (2014). Chronic lack of sleep is associated with increased sports injuries in adolescent athletes. Journal of Paediatric Orthopaedics, 34(2), 129-33
@John Smeton