The Old Hag has been documented since ancient times. Modern research has found that approx. 15% of the adult population worldwide experiences at least one Old Hag attack. Some individuals suffer repeated attacks over a limited period of time; others have repeated attacks for years (I'd fit into that category). Science has proposed no adequate explanation for what causes the attacks and why certain people experiences them.
Characteristics of attacks vary, but some are common to most incidents. The victim awakens abruptly, feeling an invisible weight pressing on the chest. He or she tries to move, struggle or scream, to no avail. The attack ends ends just when the victim is on the verge of passing out.
On a personal note I've been experienceing sporadic attacks for over six years now, and let me tell you, it's scary shit. I can't add much more to what I've already written except that it seems to run in some familys (my father and severl of his brothers and sisters have experienced repeated attacks over the years. A friend of mine had an aunt who was a psychic, according to her my attacks were being caused by spirits either created or drawn to me by powerful negative emotions (I was in a pretty nasty depression for a few years). She suggested I put salt in the four corners of my bedroom. *shrugs* It didn't work.