I'm sure this happens a lot more than we'd like to believe. Fairly common on night shift to call aircraft for 5 to 10 minutes before getting a response. I realize that they could be on the numbers or on company freq some of the time, but...
We had a guy a couple of years back that ploughed straight through Pucka military live firing zone. He was half way through before he responded to calls and then quickly apologized for dozing off. I bet he wished he'd given himself a minute to recover his senses before admitting that over the air!