I had a BOS-SIN where I took the turn from NRT back several hours later... it was a good 4-5 hours in SIN before heading to the gate for security screening so plenty of time for a shower and nap in the lounge since the flight arrived around midnight and departure was 6am And, didn't actually have to exit security/immigration, just stayed airside and had my return boarding pass printed out ahead of time at the skyclub in NRT. lol.
Was funny getting a look from the departure gate security when they saw I didn't have a SIN passport stamp, but apparently mileage runs were common there so they were used to it. Much more enjoyed staying there for a day however and touring the city and the wonderful food.