I don't see that this has been posted before...
NRT has a very good "pay by the hour" hotel inside the departure part of the airport (Inside security). I'm not sure which terminal, but it is the one that NW flies in and out of. It is only 500 yen per hour with no minimum, and each room comes with a bed and shower. If you're just looking for a good nap on a layover, I definitely recommend it.