Originally Posted by Guy Betsy

The Crowne Plaza Airport Hotel is located outside immigration and customs. So you'd have to clear both to access the hotel.

http://www.viral-life.com/crowne-plaza/Changi/

You can, instead opt for the transit hotels but 12 hours might be a tad too long to stay inside an airport. But you don't have to collect your luggage nor clear immigration here.

There is one Transit Hotel within each terminal.

http://www.airport-hotel.com.sg/