SIN is perhaps the nicest airport to sleep in that I've ever been to. I tried to book the transit hotel a few weeks out but it was full (everyone's advice there is to book EARLY.)

However, there are (free) comfy sleeping chairs, essentially recliners, in several quiet, dark, out of the way places in the airports. A pillow, eye-covers, and earplugs can make for a sleeping experience roughly equal to domestic first on United: but with vastly better legroom!

Just make sure you don't exit security unless you've already got an onward boarding pass in hand: the sleeping lounges and the transit hotel are both *inside* of security.

Note that the areas outside of security are vastly less pleasant and comfortable than inside, and the UA ticketing desk doesn't open up until a few hours before the morning flights.