3. How and when does this occur
When a person goes to sleep, they go through phases. The entire 7 or 8 that you are snuggled up in bed is not one whole phase. There are actually 5 phases that make up the sleep cycle, and it can take up to 90 minutes to work your way through all of them. During the first four stages, you experience non-rapid eye movement sleep. A person does not move much during this type of sleep, but their muscles are still functioning. As seen in the name, there is no eye movement that is associated with this type of sleep.
When a person goes to sleep, they start at stage one which is a very light sleep. They then process down the stages, and as such, the sleep gets a lot deeper. If a person has reached the 4th stage, it will be incredibly difficult to wake them up. There is not much dreaming that goes on during these sleep phases.
The last phase of the sleep cycle is characterized by rapid-eye-movement sleep. During this phase, the only part of the body that is able to move is the eyes. The heart still beats, and the lungs still move, but there is no other voluntary movement. The eyes tend to dart back and forth for no apparent reason. It is thought that this is due to people following images with their eyes while dreaming. This is where the majority of ones dreaming takes place.
Sleep paralysis can happen either when one is falling asleep or waking up. It is when a person has difficulty moving in or out of the rapid-eye-movement phase. Sleep paralysis while falling asleep is hypnagogic, and if it is experienced when waking up, then it is hypnopompic.