Those shifting, spiral-like patterns that are intermittent throughout the graph, I think those are his holidays (most occur at august), where he, as so many others "like us", turn his night and day around :)
There's some similar charts here: http://www.supermemo.com/articles/sleepchart.htm
He seems to have many projects but not much with fixed dates. I can relate, after six months of internship where I always got up at the same time and went to bed at the same time, the, uhm, flexible nature of studying immediately wrecked my sleep cycle.