While I don't take separate sleeping clothes, my base layer top and bottoms are what I use to sleep in. If it's cold in the morning, I leave them on until I'm ready to start hiking. If it's cold in the evening I may put them on well before bedtime. If it's really warm weather, the base layer may be strictly for sleeping. However, at the high elevations where I like to backpack, the nights are usually pretty cool if not downright cold, so I'm usually wearing that base layer in camp as well as in bed. It has to be awfully cold (well below freezing) before I wear it for active hiking, though.
One thing I definitely don't want to do is crawl into my sleeping bag wearing damp or wet hiking clothes. They go into a plastic bag so the moisture won't affect my sleeping bag insulation. While they are still wet in the morning, at least they are warm!
I guess you could call my base layer "semi-separate camp and sleeping clothes." That's a lot different than taking a separate layer worn only in the sleeping bag, because the base layer is part of my total insulation package.