Sleep is vital to balancing our hormones
Sleep is important to balance hormones that control hunger, metabolism, and weight gain/loss.
Sleeping has also been shown to enhance human growth hormone (HGH) secretion, which is our body's natural anti-aging hormone.
HGH is also important to the development of lean muscle. If you Google around you can find a lot more info on HGH.
For more detailed information about sleep and how it directly affects our hormone balance can be found in this article.
Basically, if you don't get enough sleep, your body won't be able to properly balance your hormones and your body will be all 'out-of-whack'.