Sleep paralysis by definition is the inability to move during a period of REM (Rapid Eye Movement) sleep but is always temporary. It usually takes place while you are waking up or falling asleep. The key feature of sleep paralysis is that your mind is awake but your body is still asleep. Experiencing sleep paralysis can last like a normal dream and is usually a frightening experience but not anymore then a few minutes. Everyone experiences sleep paralysis at least once but may not get the hallucinations.
Sleep paralysis may seem like an unusual phenomenon but actually it happens every time we go to sleep as it is very natural to us. The reason it actually happens is safety as during this period you are dreaming so your body would move if it wasn't for the sleep paralysis. If your body did move you could easily damage yourself.
There are many misconceptions about sleep paralysis and these are usually around the hallucinations which may occur. These hallucinations are caused by the brain still using the part of it which induces dreams, many of the famous hallucinations are the hag, shadow person, succubi and inncubi. The last two are featured heavily in paintings from history picturing the phenomenon. These hallucinations like dreams feel as if they are hurting you or effecting you physically but that is not true, this leads to question the myths on the two types of demons. You can make up your own decision on them but bare in mind there is little about them other then during sleep paralysis. During sleep paralysis people also report being attacked by strange monsters but they will always be a dream so be wary of people that make such claims.
Sleep paralysis is not something you will want to go through but you can avoid it by sleeping a healthy length of time. It can occur though if you have an irregular sleeping pattern, naturally or artificially. Another cause is age but being more common in teenagers or young adults due to the hormone inbalances within the body.
Narcolepsy is a disorder which causes the person to suddenly and unexpectedly fall asleep suddenly. I am no expert in this disorder but people who suffer from this often experiance sleep paralysis.
Sleep paralysis is also used as a stage in the controvercial ability of astral projection, it would also give an explanation to the OBE but it is your decision to make.
If you want to have a less percentage of experiencing these hallucinations, prepare for sleep paralysis. It could happen anytime, so if you wake up and you can't move, just think "darn sleep paralysis!" and think funny/happy thoughts. The only reason you experience these "demons" is because you know about them, and your still dreaming, so you can change them. Hope it helped.