There are five states of sleep. The first has the presences of theta waves in the brain and is called a light sleep. The second is where there are sudden spikes of high brain waves. The third starts shows the presence of delta brain waves and is also where your muscles relax. The fourth stage shows all delta brain waves and the body is in a very relaxed state. The fifth stage is REM where the brain waves are less intense and where vivid dreams occur. NREM sleep happens in the first four stages. In these, the body repairs itself, strengthens the immune system, builds muscles and regrows tissues. REM happens in one stage. It is where you have dreams that you can recall when you wake up. In this stage, your heart rate and breathing increases.
