My baby sleeps in whatever temp we do. In summer his room was cool with the AC, as was ours. Now it's cooler, so no AC, and it must be about 19-21 in there at night. He sleeps in thin long sleeves/pants cotton and a 2.5 tog grobag. When the temp drops more (and it's ridiculously cold in these apartments), he'll wear a fleecy PJ under his sleeping bag. Last year, I put a standing gas heater outside our bedrooms (they are adjacent to one another) so some heat would come in but not too much. I'm pretty sure in the winter it's about 15-19 in his room. Very chilly, but I don't want to use a heater in there.