I've never overslept for an exam (class yeah, but not exam), and I've woken others up so they don't oversleep 🙂
My freshman year, I didn't have an exam, but my roommate did. I woke up when her alarm went of and headed back to sleep. About 30 minutes later, I realized she hadn't gotten up so I yelled at her to wake her up. She barely made it.
The other story is from a few weeks ago. I am generally up and out the door before my roommate now wakes up because I go to the gym, so I didn't think anything of it when he wasn't up when I left. Then about 10 minutes before our exam I realized he wasn't in the classroom, so I sent him a text that according to him, woke him up. He was about 5 minutes late, but luckily that particular professor didn't care about people being a little late 😳