This study compared weight loss between two groups – one group got 8.5-hours of sleep per night while the other only got 5.5-hours of sleep per night. Both groups underwent the same diet and did not partake in exercise. Both groups lost the same amount of weight but the 8.5-hour group lost more weight as fat and retained more muscle mass. Sleep is incredibly important for preserving muscle mass during calorie deficits.
Reference: Nedeltcheva, A. V., Kilkus, J. M., Imperial, J., Schoeller, D. A., & Penev, P. D. (2010). Insufficient sleep undermines dietary efforts to reduce adiposity. Annals of Internal Medicine, 153(7), 435-441.