While you are sleeping, your body is repairing itself. Insufficient sleep can lead to inefficient repairing and a sluggish system. Plus, if your muscles are sore and achey, a reluctance to exercise the next day. Not to mention the possibility of being grouchy and self-defeating!
Make sure, therefore, that you get enough sleep!
Source: www.geograph.org.uk - Sleeping Statue; The Lost Gardens of Heligan