If you don't sleep 8+ hours every night: Matthew Walker - Why We Sleep
I have read both recently, and there is a lot of overlap, but the Circardian Code goes beyond sleep into nutrition and execise, as well.
Satchin said that there were no (genetic) sleep types among humans (who prefer to get up and go to bed early, or late) referencing a paper in which a group of students went on a hiking and camping trip where they all synchronized to the same rhythm after a few days of exposure to all natural light.
Matthew, on the other hand, says that there are definitely variations in sleep time preference in humans, with a genetic base and recommends to find out so that the ideal time windows for work and sleeps are chosen. Does not give a reference, sadly.
Not sure what to make of it, really.