Quote Originally Posted by SouthernBelle View Post
When I was younger, I used to clunk out for no apparent reason. Tested for lots of stuff. Nothing was ever resolved for sure, but best guess is low blood pressure. If it's already low, when you stand up it drops even lower as do you.
My dear partner had the same problem for a while. Missed a lot of school because he was always sleepy. Despite all the efforts, I don't think he ever received a definitive diagnosis.

He does have really low blood pressure and heart rate though (just like his dad I think).