Not had it for a few months now, but i did go through a period where i had it every second night or so, for around 2 weeks, its awful. OFten had it 2-3 times a night as well, just drifting off to sleep then BAM, it hits you.
Think it was mostly stress related, it must have been, as i have been fine for a few months now, but the worst thing is, you dont even know you're stressed :S