Around the time my brother and I turned 15 and 16, we started doing this all the time. We grew up sharing the same room (the same bed when we were very little), and we would take naps together all the time. I sucked my thumb a lot in my sleep. One day when I was 16, I was taking a nap with my head on his lap and it just kinda happened. We still do it all the time, among other things.