In the Annals of Internal Medicine, researchers took 10 overweight adults, put them on a moderate calorie restricted diet and then forced them to sleep only 5.5 hours, or permitted them to sleep 8.5 hours a night. This is obviously a very small study, but since they kept the subjects in their sleep lab for 2 week blocks of time, the quality of the data is excellent, making this worth talking about. Furthermore, they gave the subjects two days up front of the same 7 hours of sleep and then did a crossover. This is where they gave the subjects a break for a few months and did the experiment again, but now the 5.5 hour group was given 8.5 hours and vice versa. This way, each person was compared to themselves.
They found that everyone lost weight (remember they were all restricted on calories). But, those given 8.5 hours of sleep lost more weight from fat loss, and the 5.5 hours of sleep resulted in more weight loss from non-fat (ie: muscle) sources. They measured this through body composition, but they also measured blood tests which confirmed that those with more sleep were burning fat, not muscle as they lost weight.
This is one of those studies that proves a concept, that sleep is an important part of your health, even when it comes to things like weight loss. Clearly, this is a small group followed for a short period of time and we cannot draw long-term conclusions about these findings, but it is an interesting observation and perhaps something to consider if you are trying to lose weight (just remember that everyone was still eating a calorie restricted diet).