Sleep is SOOOO important! Your Body Does Incredible Things When You Aren't Awake. Exercise stresses the body - it is only a positive stress if you give the body the tools (sleep) it needs to recover and get stronger.
REM cycles are a key factor to the different stages of sleeping. Your body undergoes various stages where your brain waves, body temperature, dreaming, etc. These stages are important for how you sleep and what your body does.