Sketching on autopilot

Do you know that uneasy feeling when you've been driving along a winding country road at dusk and you suddenly realise that you have been on 'auto pilot' for what seems like the last 20 minutes?
You've been wandering in your head, following a thread, drifting from one idea to the next. You have a vague recollection of having negociated a junction, overtaken a tractor and trailer, and although it was you that was driving, it's almost as if you were there as a passenger.

Sometimes it happens when I'm drawing.