They do two things at once, are permanently distracted and have the most annoying habit of bombarding you with questions without listening to the answer. The neural pathways of the polyphasic type are like a Los Angeles freeway after an earthquake, with signals going right, left and centre.
Though medics see polyphasism as a psychological defect, those who have it argue to the contrary: they can talk and listen at the same time, type and read. This 'A' type personality doubles up on all activities (why just sleep when you can sleep and learn French at the same time?) There is help: stress counsellors have all kinds of useful exercises, like teaching them to do one thing at a time. But as sufferers think anyone who can't move as fast as they do is weak (or stupid), the success rate is low