Hi Remus
As Dick says, the word 'normal' doesn't seem to have much meaning in AD vocabulary, but there are lots of similar symptoms & behaviours which crop up again and again.
I can only really speak from observation of my Mum (and a little generalisation from what I have read here) but my impression is that it is the inability to concentrate or focus on things for any sustained time which causes the napping.
A few years ago, my Mum always had the radio on all day; plays, current affairs programmes, gardening, 'magazine' programmes, comedy shows. Now she'll say something like "Oh, I'll listen to that XXX, it sounds interesting" (from the radio times guide), but within 10 or 15 minutes she seems to have lost the thread of it (if indeed she has remembered to listen to it!) or she'll get up to make a cup of tea and forget she was listening to it, and do something else. Ditto crossword puzzles, books and other interests.
I think it's the struggling brain function which gets 'tired', rather than the person.
Best wishes