A 2018 study sheds light on the impact of sleep on our ability to improve our body composition. Researchers divided subjects into two groups: One group received normal sleep while the other group restricted sleep by one hour per day for five consecutive days. Both groups focused on consuming slightly fewer calories; and the calorie deficit was equated between groups. The results are fascinating. Both groups lost the same amount of weight (and this is to be expected). However, the sleep-restricted group lost a significant amount of muscle; 84% of the weight that this group lost was muscle. The group who was NOT sleep restricted lost significantly more fat; 80% of the weight lost was fat mass.
A 1994 study demonstrated that the amount of reps performed and the amount of weight lifted on multiple joint exercises (leg press, chest press, pull-down for example) reduced when in a sleep restricted state and that sleep restricted subjects reported higher perceived exertion. Subjects thought the same workout, with the same weight and the same reps was much harder when they lacked sleep.
In a brand-new, 2020 study, 23 subjects received a sleep education intervention; they learned about the importance of sleep and how to improve the quality of their sleep. Then they engaged in a 10-week strength training program. A comparison group who did not receive the sleep education performed the same 10-week strength training program. The results? Both groups experienced similar increases in their lean muscle mass; however, only the group that received sleep education reduced their fat mass.