They treat flights from Singapore to Malaysia differently at the Changi Airport. You have to check in at different areas and only two hour before the flights. While this does not imply that a through-checked luggage will be subject to problems, I will view that as a red-flag (it might be held somewhere for a long time and may not arive with you the next day). I think there are two safer options (1) enter Singapore and spend the night in a downtown hotel. The immigration is painless and the cabs are cheap. You also will have more and better choices of hotels. (2) Try to get an immediate flight to KL, if any is available. The KL hotels are much cheaper than those in Singapore (e.g., I paid 50%more for Singapore Marriott than the JW Marriott in KL).