The Midwest has hot, muggy summers. Humid doesn't even begin to describe it accurately. Luckily, most of us Midwesterners have a/c. We don't like to crank ours up too high though, so we get a lot of use out of our ceiling fans. And the best thing ever for summer: chambray sheets.
Chambray is my favorite summer fabric - it always feels cool against your skin. And it somehow holds the fresh-from-the-laundry scent longer than other sheets.