All too well! Had one the other day actually... Yuck! While I can still function when I have them (get up, get food, take painkiller etc.) they will sometimes wake me up in the middle of the night and I usually don't go to class when I have one because concentrating is extraordinarily difficult! Can't really pinpoint one cause although I think stress and lack of sleep might be two potential causes. Luckily if I catch them early, take a painkiller then sleep for an hour it will usually kill it. It's when they persist that's the worst! Yuck yuck yuck...