I've had a couple of 10-12 hour layovers at ICN in the past year. My flights had me arriving early morning (~6:00 AM) and departed late afternoon. On both occasions, I booked a room at the transit hotel in the terminal for 8-10 hours, allowing me to get some sleep, take a shower, relax, and have a meal. I found this to be a good way to break up the trip and be better rested when I got to my final destination.