I was on transit from Singapore to Guam (next morning flight), and thought initially of staying within the transit area. But in Narita, they don't let you stay overnight in the transit area. So I had to go out via customs and immigration and wait in the main hall.
There are nice comfy chairs, but the security there don't let you just sleep anywhere. After midnight, they round you up and everyone has to sleep together at the groundfloor area in a corner with the security guy looking after you. That was fine, except that (1) they kept interrupting my sleep TWICE to ask for my name and particulars. (2) at around 1pm, they switched off the heater and lights, causing the temperature to drop (it was around zero outside), and I couldn't sleep.
Wouldn't mind sleeping there again, but next time, bring warm blankets!