1) Leptin levels drop. Leptin is a hormone that influences your metabolism. When leptin levels drop, you burn fewer calories.
2) Ghrelin levels increase. Ghrelin is a hormone that influences appetite. The higher the ghrelin, the more hungry you will be.
For obvious reasons, the combination of burning fewer calories and eating more because you are hungry makes it tough to lose weight.
Many studies over the last 10 years have shown a decreased metabolism and/or increased energy intake with sleep deprivation.
So, how much sleep do you need? Most studies show that 7 hours or more are what you are looking for. I have also found this to be true with my own clients.
Take a good look at your sleeping habits. Sleep deprivation can often be a little thought of, but very important component of your weight loss program.