there could be a lot of reasons you "drift off", physical or emotional. I would worry less about trying to read the therapist's mind - especially since they are trying to read your mind at the same time! or at least trying to get a "feel" for what you are experiencing by how it makes THEM feel - so the more you can just concentrate on and SAY whatever you are feeling or thinking - even if it's, "I'm drifting off now" - the better and faster therapy will work.