It's pretty rare for our nighttime lows to be above 72, even in the Summer, and I live in a fairly hot part of the US (though not the South proper). Even when it's in the high 90s or low 100s during the day, which is typically only a few weeks a year, usually it still gets down to 75-78 at night. We're in the high-80s this week and nighttime lows will be as low as 58F.

Spring nighttime temps are more like 35-55.

The main trouble's the damn humidity. A very-humid 72 still feels gross. And maybe I could sleep like a baby on a night when it gets down to 60, but if that's a spring or summer night I'll wake up nasty and covered in sweat unless I've got some serious dehumidification going (that's what happens when I go camping—might be cold enough to sleep well, but unless it's Winter or late Fall the high humidity still has unpleasant effects). Plus you get mildew and mold problems if you let a modern, very-sealed-up (so, efficient for AC) house get very humid too often, so it's better to just dehumidify with the AC even if the outdoor temp matches what I'd like it to be indoors.