Why must we sleep? It seems like a fairly simple questions, but if that were true, you would assume scientists would have more than a couple of theories to answer this mystery. However, they do know what goes on within our body during various stages of sleep, and they can explain where dreams fit in to our nightly slumber.
The first theory pertaining to why we need sleep is called the repair and restoration theory. This theory states that our bodies and brain need time to rest, but is often disputed by the fact that our bodies can be rested by just laying down and further more, our brains become very active during sleep.
The second theory is known as the evolutionary theory in which the belief is that humans have always slept and our bodies are programmed to do so. This theory would also account for the fact that different animals need different amounts of sleep (a bat needs approximately twenty hours of sleep a day, while a horse needs only three or four).
After we have finally fallen asleep for the night, our bodies enter stage 1 of sleep. In this stage, brain activity is unsynchronized. As we enter stage 2, our breathing , heart beat, and brain activity become slower. Next, in stage 3, our breathing, pulse, and brain activity is slower yet, and brain activity is synchronized. During stage 4 of sleep, our breathing, pulse, and brain activity is the slowest, and brain activity is highly synchronized. Stage 3 and 4 are predominant the first half of the night, and stage 1 is usually replaced with REM sleep (the stage of sleep we are in the 20-25% of the night). During REM , our eyes move rapidly and our dreams are very complex and vivid. It takes 90-100 minutes to get from stage 1 to stage 4, and back to stage 1.
Dreams can occur in other stages of sleep other than REM, but are usually more vague and not as bizarre. We can incorporate external sounds into our dreams such as a telephone ringing of thunderstorm.