Mirroring what others have said, often even heavy sleepers have sounds there sensitive to that wake them up immediately. I had someone crash on my couch one night. I knew they had to be up in the morning for work, so I set an alarm for myself to make sure they got up too (this is right around when smartphones were starting to become a thing, and it wasn’t really common to have alarms set on your phone, hence why I was going through the extra effort for them). I woke up and went over to try and wake them up. First it was a soft nudge in the shoulder, then a harder one, then a shake, then grabbing them by both shoulders and aggressively shaking them. I literally stopped to check their breathing and make sure they hadn’t died on me. My roommate came out during this and was also laughing and wondering if they had died. I really had to use the restroom, so said I was going to do that and then come back and give it another go. While I’m in there, I hear their phone go off and my roommate starts laughing hysterically. Apparently as soon as the tone went off on their phone, they sat upright, wide awake.