There are many stages in the sleep cycle, and different processes occur in each one.
In science's continued search of the true purpose and meaning of sleep, many answers can be found in the stages.
Of course, why the stages even exist is another quest in itself, but thankfully it is not something that we have to worry about.
When we are awake, our brains are using a type of wave known as the beta wave.
This enables us to think quickly and operate the way we need to.
The first stage of sleep involves a transition from beta waves to alpha waves, which are a little slower and can be achieved through simple relaxation even when not going to sleep.
It is also during this stage that the hypnic jerk can occur, in which the brain misunderstands muscle relaxation as falling, and causes the limbs to move suddenly.
Stage two is an interesting stage, as it has two unique features that no other stage has.
The first is the sleep spindle, which is a quick, rising increase in the amplitude of brain waves.
The second is the k-complex, which involves sharp drops and rises in amplitudes.
More theta waves (the next type after alpha) are present throughout the brain now.
Stage three introduces the delta wave, which is even slower than theta.
This stage is what most people consider to be deep sleep, and it is here that one cannot be woken up easily.
Stage four is identical to stage three, except delta ways are more pervasive in different parts of the brain.
All the stages heretofore have been NREM, or Non-REM sleep.
The last stage is REM sleep, which stands for rapid eye movement.
REM is also known as paradoxical sleep, because the type of wave present is the beta wave, which is the one present when we are most wide awake.
REM sleep is the most important part of sleeping, as that is when the brain does the most recovery.
One's body also becomes paralyzed in REM sleep, as dreams occur mostly in this stage, and if the body was not paralyzed, there would be nothing to stop people from acting out their dreams.
Therefore, even if you are having a nightmare, you will not move around in real life, contrary to what is portrayed on television shows.