If you're making an overnight connection, you can't clear immigration and re-enter without a boarding pass for your next flight. If you got one at your point of origin, you're OK: get your bags and come back in. If you didn't, you can't get one at SIN check-in counters until the day of your flight. If it's after midnight, you're still OK since it's now your flight date - if your airline's check-in counter is open. Failing the above, you have a few choices:

1. Carry enough stuff with you for overnight and check your bags through to your final destination. Best if your connection allows that.

2. Carry enough stuff with you for overnight, pick up your bags at the airline's baggage office the next morning, get your boarding pass, and re-enter.

3. Get an airport staff member to escort you to your bags and back in. They will let you retrieve medications and other absolute essentials. They will most likely not let you get PJs and clean underwear, even if you're already there for the meds, though this may depend on the person who escorts you.

4. As posted, stay elsewhere. Less convenient, and (unless you book a really long block at the TH) more costly.