Sleep does a lot of different things for the body. The physical stuff is fairly obvious; your body heals injuries, fights off whole-body infections, rebuilds muscle after major exertion, etc. As for the mental aspects of sleep, there are many and varied parts. Sleep plays a huge role in memory during which time the hippocampus sorts memories from the day and either makes them permanent in long-term memory or leaves them in short-term to be pushed out later by something else. As for emotional fatigue, it is, in part, an illusory sensation. Should you suddenly find yourself being chased by a tiger, you'd discover that your body is fully capable of performing the necessary activity to preserve itself. However, stress and emotional fatigue do sap motivation and make it appear as though certain activities would require vastly larger amounts of effort than is truly required.
