Thanks, that's really interesting, including the Criticisms section. I think that there must be a mix of psychological and physiological factors at play.

Not sure how directly relevant this is but I used to get dead-tired if I had any less than eight hours of sleep. I had spent most of my life hearing about how I "needed" at least eight hours. Then I had a period of insomnia where I would regularly get three hours a night which challenged this deeply-held belief. My functioning was impaired, of course, but it was comparable to how I functioned on 7 hours of sleep prior. Now I can sleep six hours a night without problems, having broken through some psychological barrier.