Sleep paralysis isnt very well known, however it is a legitimate sleep disorder. I know a few people, including myself who have experienced it, and it is not a pleasant disorder. In sleep paralysis, you lay completely awake, but unable to move or use your body in any way. These symptoms can vary in widely in duration, but usually lasts a few minutes. In the instance where I experienced it, I could not move, I was essentially trapped within my mind. This disorder can be caused by many things, including stress, lack of sleep, and inactivity.
As highlighted in the prompt, Sleep Paralysis (also called Hypnopompic paralysis) may be a simple explanation for some pretty bizarre occurrences. People report being abducted by aliens or having horrifying experiences and hallucinations. By using the principle of Occams Razor, we can debunk a lot of these stories. Research has proven that during sleep paralysis, hallucination can, and often does occur. Some individuals understandably would not be able to tell the difference between reality and these hallucinations, because while they are happening, they seem completely real. In my experience, I never had hallucinations, but I can see how terrifying and lucid they would appear to be. This phenomenon is a great example of how a crazy occurrence can have a very simple explanation.
Sources of facts: http://www.stanford.edu/~dement/paralysis.html