I believe that the "old hag" is also known as a succubus and dates back at least to the middle ages. The male version is known as the incubus. Its thought that this was the cause of what we know today as sleeping disorders (and I suffer from one).
The succubus and the incubus are said to involve sex. That would be a different thing altogether!
What kind of sleep disorder do you have? I've had insomnia myself since I was about 12, which lately is aggravated by a sinus/migraine problem.
On the incubus/succubus experience, the anthropologist Edward Burnett Tylor in his book Religion in Primitive Culture mentions that in addition to medieval Europe, traditions about this type of experience are also found in other cultures. The succubus/incubus experience can indeed be very similar to the Old Hag experience Hufford focuses on. He observes this and suggests for classification purposes that the succubus/incubus experience is usually described as having a sexual component whereas the Old Hag experience usually doesn't, though sometimes there are Old Hag experiences which take on a sexual element so these are overlapping categories. I'd add to what he says that sometimes there are succubus/incubus experiences which don't involve a sense of paralysis, and like the Old Hag experience the succubus/incubus experience may occur both while sleeping and while awake, though it seems to be more common in sleep.