The days here are still relatively warm, but the nights have gotten downright COLD! My husband and I often wake up at night pretty chilly, even though we set the thermostat to 68 degrees (yeah, not too frugal, but I almost NEVER turn on the AC, so there) and have both a flat sheet and a comforter. While I was in Target, the other day, I sought to find a remedy to this problem, and I believe I have. I purchased a cheap cotton blanket to tuck between our sheet and our comforter. It's not too well-made, which means the weave is loose and there's a lot of space for air to get trapped, upping the warmth potential. It's perfectly sized, so it fits between the other blankets without showing, because it doesn't quite match them, color-wise.
I don't know how well it will hold up to repeated washing, as a few of the reviewers mention it kind of falls apart after washing and the hems look a little flimsy, but for our purposes, it's doing very well. After just a couple of nights under it, we're both sleeping much better and aren't freezing while in bed. Definitely a worthwhile purchase.