I am overnighting Airside in Singapore as well. I haven't tried it yet, but there is an airside hotel (actually two), and one of them get good reviews and is pretty cheap. You can book it through hotels.com and costs 35 EUR for 6 hours. They have a pool as well. Name is Aerotel. I thought paying this instead of a lounge to get a proper room is pretty good.

I you are connecting in SIN with SQ you are also qualifying for the stopover program of SQ which has pretty neat Hotel Prices that also include free entry in major attractions in SIN.