Yes we do know and accept that lack of rest, adequate sleep, build up of sleep debt leads to fatigue. But I would submit, so does inactivity which causes lethargy, boredom and impairs alertness and attentiveness.
The current line of thinking is that the act of monitoring ( looking at gauges and screens) is in itself, tiring/ fatiguing due to ' repetitive cognitive effort'.
Makes sense to me. It explains why after a long period of intensely monitoring the automatics in the cruise as weather is negotiated one can feel quite exhausted after the duty. The automatics have done it all, but it is more tiring than a normal flight.