I don't know about summers, but my parents enjoyed only turning on the heaters in winter when it got to maybe 45F indoors. Any colder and it might wake you up in the night, which is tremendously distracting, but aside from that there are no meaningful downsides to just wearing a sweatshirt. But those temperatures probably aren't common outside of Minnesota, Montana, the Dakotas, probably other midwestern areas (I'm in Michigan at the moment and it's bizarrely temperate -- people insisting coats are needed when sweatshirts hardly are).
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