I don't know why these pink fuzzy socks are so warm to sleep in but they are.  And they machine wash and dry like a dream.  I use them every night.  I have small feet (size 7.5) but I can see that they will cover many sizes.  If you constantly have cold feet, this is a good way to start.

UPDATE:  A year later and I just bought another pair.  I LOVE these warm socks. I wear them all winter to sleep over my regular socks and wash them constantly - they machine wash and dry like a dream.  The new pair seem to be a bit larger than the originals.  I have a small foot - 7.5 shoe shize - and they fit just fine but with plenty of room for much bigger feet.  One of the new ones did slip off one night - the old ones never did - so I guess that means they are larger.  Anyway, I continue to highly recommend them.