Let's review the facts:
You arrive 11:30pm. You then go to transit hotel airside to sleep for six hours at 3:30am. This put us at 9:30am.
Your flight leaves late morning; at what time? Even if it departs 11:59am IMHO you would not have time to enter S'pore, take the SMRT into town, immediately return and get process by immigration for departure and security.