A lot, I think, depends on your metabolism since some sleep cold, some hot and presumably some are "average."

My summer bag in the Sierra is usually 35 or 40-degree, always down and so, very small and light. I can supplement with a jacket and hat if I need to, which is pretty rare.

Now that waterproof down is pretty common, I'd think the more soggy AT would accommodate a down bag in lieu of bulky synthetic. Pad/mattress choice should be considered along with the bag, too.

Cheers,
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--Rick