WHAT HAPPENS WHEN WE SLEEP?

Sleep is a very active state. Our bodies move frequently, as we roll about during the night, and, more importantly, our brain activity is even more varied than it is during normal waking state.
When awake, most people exhibit brain wave patterns that can be classified into 2 types of waves, beta and alpha.
Beta waves are those associated with day to day wakefulness. These waves are the highest in frequency and lowest in amplitude, and also more de-synchronous than other waves. That is, the waves are not very consistent in their pattern. This de-synchrony makes sense given that day to day mental activity consists of many cognitive, sensory, and motor activities and experiences, and thus, when awake, we are mentally de-synchronous as well.
During periods of relaxation, while still awake, our brain waves become slower, increase in amplitude and become more synchronous., These types of waves are called Alpha waves. Activities that promote Alpha wave activity, have positive health benefits.
When we sleep, the first stage of sleep is characterized by Theta waves, which even slower in frequency and greater in amplitude than Alpha waves. The difference between them is gradual and subtle. As the sleeper moves to stage 2, Theta waves activity continues, interspersed with 2 unusual wave phenomena. These phenomena, which occur periodically every minute or so, are termed with a sudden increase in wave frequency, and a sudden increase in wave amplitude. Stages 1 and 2 are relatively light stages of sleep. In fact, if someone is awoken during one of these stages, the person will often report no being asleep at all.
During a normal sleep a sleeper passes from from the Theta waves of stage 1 and 2, to the Delta waves of stage 3 and 4. Delta waves are the slowest and highest amplitude of brain waves. There is no real division between stages 3 and 4 except that, typically, stage 3 is considered Delta sleep in which less than 50% of the waves are Delta waves, and in stage 4 more than 50% of the waves are Delta waves.
Delta sleep is our deepest sleep, the point when our brain waves least like waking. Consequently, it is the most difficult stage in which a person can't wake up easily, and when they are awakened they are usually sleepy and disoriented. Delta sleep is when sleep walking and sleep talking is most likely to occur.
Besides these 4 basic stages of sleep, another, unique, stage of sleep exists. This stage gets its name from the darting eye movement that accompany it, Rapid Eye Movement (REM). It is characterized by a sudden and dramatic loss of muscle tone, in fact, the skeletal muscles of a person are effectively paralyzed. This is associated with a unique brain wave pattern in which the waves demonstrate characteristics that are similar to waking sleep, a combination of Alpha, Beta, and de-synchronous  waves. This is the stage of sleep most associated with dreaming. When a sleeper begins to exhibit the physiological indices of REM sleep, and they are awakened, the great majority of the time they will report that they were having a vivid, story-like dream. During other stages, on the other hand, they normally do not report dreaming.