I just got back on a even keel (after a month in the US - great trip).
The SIN stop incorporates a Qatar crew change. Everybody is requested to de-plane with your hand luggage (probably because it's cleaned by a contract ground crew). Very painful at 1am - 2am in the morning. You are forced to exit out of the departure gate into the main arrivals hall, then within minutes called back through a security check and into the departure gate lounge.
All the SIN transiting passengers are grumpy and tired.