In reading a book on Gestalt Therapy, I came across a chapter mentioning that same problem, feeling sleepy with some clients, but not all. What was said was that if you are becoming sleepy it is probably due to the clients boring droning voice and content of their conversation. What the therapist did was actually say to the client, "you sound rather bored with where you are at right now, how do you feel". Having shone a light on their boredom of the area which the client was talking about, the therapist was able to help them move on to an area whch was more productive! Don't know if that is any help!

Jacqueline