What an excellent question! I often times wondered why people had to sleep when there are several other animals that actually do not sleep. They go into a rest period, but it is not the same as when humans sleep.
There are several theories as to why humans sleep. The first theory is that when we sleep it is usually at night time; therefore, sleeping is a way to keep ourselves safe. The second theory is the inactivity theory, which goes hand in hand with the first theory. During sleep we consume about 10% less calories, which means that we do not need to consume as much. The third theory I found is the rejuvenation theory, which states that during sleep we heal ourselves, basically. For example, if an animal is deprived of sleep for weeks at a time, it loses its immune system, which eventually kills it. The fourth theory is brain plasticity. The theory is brain plasticity connects to the rejuvenation theory in a way. Scientists know that sleep plays a large part in the development of a child’s brain and the learning abilities of an adult to stay sharp.
My favorite theory is my own: we sleep because it conserves energy and allows our bodies to use that energy elsewhere to keep us healthy and of a sound mind.
People who experience sleep deprivation experience the following: aching muscles, flu-like symptoms, confusion, depression, hallucinations, hand tremors, headaches, sensitivity to cold, increased blood pressure, increased stress hormones, and in serious cases death.