I'd second that you may just need a single layer of breathable material: nylon, silk, or very light wool/fleece. Just change your clothing to suit. Try a light sheet at single or double thickness, or a cheap fleece throw from your house in the backyard with your clothing to see if that will work.
I don't usually use a sleeping bag/quilt if it's above 40, sometimes lower, but that's just me. Long underwear+hat+silk liner bag+light bivy is enough for that. Adding raingear, thick socks, and a very light primaloft sweater gets me well into the thirties in midwest summer weather (where it's nearly guaranteed not to plummet temps like it can in the mountains, and where the days will almost always be warmer than nights).
If you are only on an overnight where you are certain it will be muggy, a cotton sheet--cut to fit if you wish--can be your most comfortable option (maybe not the lightest). Blasphemy, I know. Not recommended for big mountains, long hikes, the unknown, or Antarctica.