A comforter that is all bunched up down around your feet is not going to keep you warm. This condition is most likely to happen when you've put the comforter into a duvet cover to keep it clean -- an excellent idea with some potential drawbacks. While the duvet cover remains in place, the comforter (also called a duvet, hence the name of the cover) may slide down to the bottom of the cover -- and your feet -- as you move around during the night. The solution to this annoying problem is simple and quick.
Cut four 12-inch-long pieces of 1/2-inch-wide twill tape (any colour -- you won't see it). Turn the duvet cover inside out.
Fold a piece of the twill tape in half. At the midway mark, sew the tape to one corner of the duvet cover. Double stitch, since the tape is going to take quite a lot of stress. Try to sew the tape to a seam allowance (the edges of the fabric that go beyond the seam) so the stitches won't show at all from the outside. Repeat for the other three corners of the duvet cover.
Turn the duvet cover to right side out. Insert the top edge of the comforter into the duvet cover and gather a little "ear" of comforter at the left-hand corner. Use the twill tape sewn into the duvet cover to tie the corner of the comforter to the cover. Repeat for the right-hand corner of the comforter.
Slide the rest of the comforter into the duvet cover and tie the bottom corners to the comforter to the cover in the same way. Zip or button the duvet cover closed.