I read somewhere that in one experiment tapes of clients who made therapists sleepy were played to other therapists who all felt sleepy on hearing the tapes. The explanation put forward was that when clients want to conceal something they unconsciously made their therapists sleepy!
Well, I've replaced my comfortable chair with a wooden chair which seems to help. Odd, though, how I've never, ever felt tired or sleepy with most clients.
Sounds a bit daft, I know, but there might be something in that theory about therapists feeling sleepy when clients don't want to reveal something: perhaps they engage in inconsequential talk which bores the poor old therapist.