I took the first class SIN-KUL train last year about this time. The trip doesn't afford a lot of time for sleeping (or showering the next morning for that matter) and you have the added annoyance of riding the train for 30-45 minutes just to have to disembark at the Malaysian border to go through immigration. That takes a long time while you wait for the entire train to clear.

The bed was a bit narrow and I found the shaking made it hard to sleep/stay in bed. The early morning arrival was a bit troublesome too.

Only after arriving did I read about the first class busses (some have sleeper arrangements I believe) which looked like a much better option in hindsight.