If left to themselves (that is, given no external cues such as daylight or Saturday morning cartoons), humans will adjust to a 25 hour day, rising an hour later each morning. No-one seems to be sure why this is, but it seems to indicate that our societal sleep debt is at least partly genetic.

Most people require 9.5 to 10 hours of sleep a night. Most people don't get it. Sleep patterns have already started to deteriorate in men by age 35, and it gets worse the older you get.

Stanley Coren has written an excellent book, entitled "Sleep Theives" on this and many other subjects surrounding sleep. Including the workable but unwise scheme of sleeping fifteen minutes every hour or so, 'round the clock. (You start to get stupid, apparently.)