So, for an average sleep time of eight to ten hours, we go through this sleep cycle four to five times a night. In the first phase of sleep we find ourselves in a light sleep that lasts five to ten minutes. At this stage, our eyes still move slowly and our muscles slow down, but sudden muscle contractions can still occur, as if we are scared or as if we are falling. People in this stage may not feel rested if they are woken up and may not even feel as if they are asleep. In the second phase of the sleep cycle, you have successfully fallen into a light sleep. During this cycle your heart rate slowed and your body temperature dropped. You can no longer move your eyes and your body is finally resting the parts it used during the day. The next phase, the third, is also combined with the fourth phase of the sleep cycle. These two cycles together are known as delta sleep or deep sleep phase and make up a very important part of the sleep cycle. During these cycles your body repairs and regrows tissue, strengthens your immune system, and builds bones and muscles. In these cycles it can be very difficult to wake a person, and if awakened, they may feel groggy or "out of it" for several minutes. These stages are when most people have night terrors, experience sleep walking, or talk in their sleep. In the average adult sleep time, this takes up about 15-25% of sleep time. Finally there is the REM cycle, also known as paradoxical sleep. During this cycle most muscles are paralyzed, eyes move rapidly, and breathing, heart rate, and body temperature are unregulated. Vital signs show that during this phase, arousal and oxygen levels