Did a 10 hour layover in Narita recently and it wasn’t bad considering I didn’t go into the city. I had time, but it was overcast and rainy the whole day so it didn’t seem worthwhile.
Plus, the clouds and rain meant that my ~6 hour redeye from Ho Chi Minh City to Narita had turbulence for almost the entire flight (maybe two hours total of non-turbulence), which combined with my difficulty sleeping on airplanes in general meant that I barely slept at all.
So I rented a day room for 4 hours for what ended up being about US$39. I showered and then took a 2.5/3 hour nap where I was dead to the world and woke up groggy but revitalized with about 4 hours or so left, which just flew by.
Weirdly, my 5 hour layover in Narita on the flight to Vietnam actually ended up feeling like it took longer than the 10 hour one.