I have--twice. The first time was done consciously. I became ill and really needed to lie down, and for a while I could minimally respond. But then my T said it was OK if I just wanted to let myself drift off, and we agreed that I wanted him to stay with me. He was fine with it. The other time was just after a session while still in his office pulling myself together. I had been up all night with a toothache, but I think that time was dissociative in nature. He was in the outer office doing paperwork, and came back to get me and woke me up.