What I have done before is, if I took the red eye from SIN I head straight for the lounge and sleep in the massage chairs in the lounge. But I have also gone to Narita city since you have 8 hours. Mos of the stores will be closed that early but you can walk around and have lunch before returning to he airport. You will not have enough time to go into the city itself even if you do it will be really tight.