A hot cabin at night when people are sleeping in their clothes is pretty uncomfortable and feels sweaty and 'stuffy' and makes people start accusing airlines of turning airconditioning packs off with high passenger loads (they don't). It also makes people thirsty so they keep the cabin crew busy, when half of them are off asleep (well they are in BA anyway), getting drinks all night (and you really don't want to drink that aeroplane water- take a lovely bottle of Volvic). A cool cabin is far more comfortable.