Updated: Nov 21, 2020
We all know that sleeping enough every day is important for general health, but also for your fitness goals, and this seems particularly true if you’re trying to lose fat.
A study by Nedeltcheva et al.  compared 2 groups:
Group 1: subjects slept 8,5 hours per day.
Group 2: subjects slept 5,5 hours per day.
For 14 days, both groups were put in a moderate caloric restriction to lose fat. What were the results? Well, both groups lost the same amount of weight (~3 kg), but the amount of fat mass lost was different between groups:
Group 1: ~50% of the weight lost was fat.
Group 2: ~25% of the weight lost was fat.
This means that the group that slept more every night lost twice (!) the fat, which shows us the importance of sleep if you want to lose as much fat as possible without sacrificing the amount of muscle you already have.
In another study, Wang et al.  compared 2 groups:
Group 1 (only caloric restriction): subjects slept ~6h 25m per day.
Group 2 (caloric restriction + sleep deprivation): subjects had a sleep restriction for five of the seven nights of the week and were allowed to sleep as much as they wanted in the other two. On average, subjects slept ~5h 30m on the five nights of restricted sleep and 7h+ on the other two.
During 8 weeks, both groups restricted daily caloric intake to lose fat. What were the results? Well, similarly to what happened in the previous study, both groups lost the same weight (~3 kg), but in the sleep deprived group, only ~17% of total mass lost was fat mass, and 83% (!) was fat-free mass. In the group without sleep deprivation, it was the exact opposite, which means that ~83% of total mass lost was fat, and only 17% was fat-free mass.
We can take a couple of takeaways from both studies:
When dieting to lose fat, sleep will impact the ratio of fat mass/fat-free mass loss. When you restrict sleep, your body will shift its focus, and "burn" more fat-free mass instead of fat mass, which is no bueno, because it's the fat mass that we wan't to get rid of.
Strategies like “catching-up” with sleep on the weekends, for example, might not be enough to reverse #1. On the second study, we saw that even when subjects slept as much as they wanted on two of the five nights of the week, the sleep restriction on the other five nights still had negative impacts on the quality of weight lost.
Now, it's also important to point out that both studies have limitations, being the major one the fact that the participants didn’t train with weights. If they had, fat-free mass losses wouldn’t be as high. However, I still think sleep is very important if you want to lose as much fat as possible, while retaining as much muscle as possible.
If you're trying to lose fat without losing the muscle you work very hard to gain, make sure to keep your sleep in check. Have, at the very least 7 hours of quality sleep every day (8h+ would be perfect!).
 Nedeltcheva AV, Kilkus JM, Imperial J, Schoeller DA, Penev PD. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann Intern Med. 2010 Oct;153(7):435-41.
 Wang X, Sparks JR, Bowyer KP, Youngstedt SD. Influence of sleep restriction on weight loss outcomes associated with caloric restriction. Sleep. 2018 May;41(5).