I was interested in this \link{http://www.bbc.co.uk/radio4/womanshour/2004_45_mon_05.shtml\Womans Hour item} about why some people like to wake up early and others prefer to wake up late and stay up late at night.
It's to do with cortisol, apparently, the hormone which wakes you up and gets you going. There's a whole spectrum between lark and owl depending when your cortisol peaks.
So what are you? I'm an owl. No matter how tired I've felt in the afternoon, and how I've longed to get into bed, I perk up again at about 9pm and wonder why others are flagging.