I've got some of the M65 Pant liners for cold weather lounging. I had not thought about sleeping in them, although there is no reason that I couldn't. They are cut short to be worn with boots and they are cut very generously, so they aren't going to fit in any close fitting pants. Somewhere, I've got an old surplus mountain shell pant that should work with these liners. They are made to button into the cold weather pants that they were made for. This winter hasn't been that cold around here, so they haven't been outside yet.