Fatigue or sleep deprivation. A long while back my co-pilot fell asleep on base leg into Gatwick. Early morning & I had to reach over to his shoulder & shake him awake. Obviously I was PF, but not sure how much PM I got. However we landed safely. (737-300) taught me a lesson that when the human body says enough is enough it will go to sleep whatever. We are all human maybe some understanding is in order.
(co-pilot had been awake during the descent)