Let's face it, your training could be on point, but without enough sleep, are you really building any muscle?
Getting enough sleep is critical for helping an athlete to maintain optimal health and well-being, as well as time for recovery of damaged muscle fibers caused by a workout session.
1. RECOVERY
The body heals itself the most during sleep. When one sleeps, growth hormones (HGH) are released by the body into the bloodstream to repair damaged tissues.
This helps build muscle and greatly reduces the chances of injury as your muscle tissues will be fully repaired and good to go for another intense session.
If one lacks sleep, growth hormone production is significantly reduced and therefore optimal muscle growth will not be possible.
2. ENERGY
The energy one has after a good night sleep is like no other. According to a 2016 study by Echeverry discussing the importance of sleep on performance and recovery, when asleep, your body's ability to produce Adenosine Triphosphate (ATP), the body's energy molecule, is optimized. 
3. WEIGHT MANAGEMENT
Sleep patterns can influence the production of ghrelin, a hormone that stimulates hunger. According to Swartzendruber (2013), people who are sleep-deprived tend to have higher ghrelin levels, more hunger, and less feeling of fullness compared to those who get seven-to-nine hours of sleep.
You will likely eat more than you usually do, so you will consume more calories than you burn, resulting in weight gain.
So if you want to get that last rep in, you better have a good night sleep or your body will not even bother.
Reference
Luisa Echeverry ’16, M.S. in Exercise Science Major: Exercise Science. “Importance of Sleep on Performance and Recovery.” Adelphi University, Adelphi University, 27 June 2020, https://www.adelphi.edu/news/the-importance-of-sleep-on-performance-and-recovery/.
Kris Swartzendruber, Michigan State University Extension. “The Importance of Rest and Recovery for Athletes.” MSU Extension, 21 Jan. 2013, https://www.canr.msu.edu/news/the_importance_of_rest_and_recovery_for_athletes.