There are five stages of sleep, four stages of Non Rapid Eye Movement (NREM) and one stage of Rapid Eye Movement (REM). In stage one our brain waves slow down and enter a pattern of theta waves. There are eight cycles per second. During this we may experience brief dreamlike images that resemble vivid photographs.

After a half an hour we enter stage two. Brain waves slow down to about four to seven cycles per second. Sleep spindles interrupt this stage. They have a frequency of twelve to sixteen cycles per second. We also respond to external stimuli, such as the bell at the end of a class.

Once we enter stage three the delta waves slow to a frequency of one to three cycles per second. Then stage four. It is the deepest stage of sleep, and the one that's the hardest to be awaken from. The waves slow down to .5 - 2 per second.

After a half and hour of stage four we go back upward through the stages until we enter REM. This is the time during which most of us dream. We tend to go through all the stages five times a night.

Nobody is 100% sure why we need sleep. There are many theories made by researchers. Some believe we need sleep to help rejuvenate a tried body. Others say we need sleep to help our mind and memory. Researchers have found that those deprived of sleep have trouble paying attention, learning, and remembering things. Sleep is very vital for our survival.