A humorous (albeit terrifying) anecdote:

My girlfriend's brother is a doctor, and falls asleep very easily. He told us that during his residency, he once fell asleep during heart surgery while holding open the patient's chest (with some kind of crowbar-like medical instrument whose name I forget). Fortunately he maintained muscle tension so nothing bad happened. The surgeon noticed he was sleeping and told him to take a walk.

He also falls asleep driving, standing on the bus, at the dinner table, etc.