If you do pick up your bag and go through customs into the arrivals terminal, there is a cafeteria upstairs that has seating available at that time of nite. There is also a sushi bar type restauarent next to starbucks on the main departure floor that i've seen people passing time at, even after hours.

If you stay in the transit lounge, there are attendants that will pick up your bag from the luggage claim area and place it off to the side with other unclaimed luggage. Of course there is always some risk, but I would think your bag would be more or less safe there for a couple of hours. You emight even ask one of the staff in the transit lounge to check with these bag attendents to be sure.

Last time through there my bag wound up being "claimed" by these attendents, had to show them my ticket bag check to get it from them. good luck, changis not a bad place to while away some time if needs be, overnite it's pretty quiet until around 4 am when early departure time flights start to show up.