How can I stop knotting up my hair while I sleep?

Mostly every morning I wake up with a huge knot in my hair that I (unconsciously) twirled in the night. I used to do this when I was a child as well and I haven't seemed to grow out of it quite yet (I'm eighteen now). When I untangle the knot(s) the following morning, my hair is very frizzy and brittle, which means the only way to do my hair that is with a flat iron. My hair is naturally very shiny and wavy and I adore leaving it as such whenever possible, so please don't suggest I put any serum or product in my hair; I simply want to stop knotting it up in my sleep. Thank you!!