in the late '80s a pilot fell asleep in a Trislander between Amsterdam and Southend
In the late '60s this pilot fell asleep in a Prentice, very early on a lovely but very hazy morning, between Athens and Rhodes, at 8000ft. He woke up after a few minutes, with the aircraft rumbling gently along, wings nice and level, and spent a long time sleepily pondering whether to turn to avoid the AAA exercise fire he could see sparkling miles ahead, well above the aircraft. At about 1,000 feet he woke up properly when the penny dropped and he saw that the sparkling was the sun reflecting in the choppy sea, and that he was in a shallow dive to oblivion.
I don't know if the exhaust fumes which came into the cabin by many routes aggravated the normal lack of awareness, but it's possible. Not hypoxia, though, in a Prentice.