When we got Hyrum the Buzz bed, we told him it was his Big Boy Bed, and Big Boys stayed in their beds.
On a rare occasion, I remind Hyrum that he is getting to be a Big Boy and should therefore do x.
Grandma reports that she asked him why he hasn't been taking his naps anymore, and that he responded something like "No nap. I Big Boy."
Where he would get the idea that Big Boys don't nap, I'm sure I don't know. He still needs one and takes one within minutes of setting out in the car if the time is right, but (big) boy he struggles and resists most days now!
(On the other hand, we got him some shoes that have Buzz Lightyear on them this week and yesterday he told us he wanted his Baby shoes... Curious.)