I know you've seen this link, Fizz, but for others who may be open to more than one answer, here's a link.That’s the scientific explanation and I’m glad it’s the reason for most everyone. However, there is at least one other reason for this. I also do have to say that I do have regular sleeping patterns. I go to sleep every night around 12:30-1 and wake up at 8am. I’m never slept deprived.
A different look at Sleep Paralysis