Unfortunately it has happened plenty of times Razoray. I guarantee you that many pilots would privately admit to you they had come close to nodding off. There have been several recent cases where it is understood this happened.
If this happened (if), it is not really the pilots fault. Humans are poor at overseeing things that mostly go right and require minimal intervention. Lack of mental stimulation, many hours of monotonous routine, night, chronic disruption to circadian rhythm.... Plenty of middle ages pilots will have obstructive sleep apnoea and chronic sleep deprivation. Honestly, it is amazing it does not happen more often (or maybe it does).
Sleep deprivation was cited as a significant contributing factor with Three Mile Island, Chenobyl, Bhopal, Exxon-Valdez.......
Falling asleep is the highest cause of single vehicle highway crashes at night. And driving requires a lot more effort (= mental stimulation) than flying. Plus these drivers are not driving 14 hours non-stop and coping with disrupted circadian rhythms.
By no means am I saying this is what happened. However it is certainly a possibility, and it does explain some issues prior to the crash. Therefore it is a possibility that must be considered unless it can be ruled out by the CVR.