Here’s an interesting device I tried last night with quite pleasing effects. Get about a 3 foot wooden dowel and mount a dildo on one end so that the total length is about the distance from your ankles to your anus. On the other end screw in about six inches of ¼ inch threaded rod, add a wing nut and a washer, and screw the nut down almost all the way to the wood. Tie your feet together using quick links or a short piece of chain, then put the threaded rod through the center link. Insert the dildo at the other end into your favorite orifice, then start screwing the wing nut out until it forces your legs to straighten. Lie back and secure your hands so they can’t reach below your knees. Now you will not be able to bend your legs without forcing the dildo deeper inside you, and the dildo cannot come out on its own. It's even more effective if you bind your knees together with the strap going around the dowel.
I tried sleeping like this most of last night and every time I moved I was sternly reminded that I should keep my legs straight. I added another twist by securing my hands to a rope in front of me and attached to the upper part of the dowel so that whenever I tried to move my hands closer to my head that too would pull it deeper into me. Overall a very stimulating, if not restful night.