I've been fighting insomnia since early december. It has come and gone for most of my adult life, but recently I noticed that google was in a position to make such diagnosis.
Google reader features a trends panel with a tab that shows your usage by time of day. You can see I don't generally hit the net until the rest of my family has gone to bed, and that most nights I'm done by 3 am, though it is not uncommon for me to up til 6 am. I have noticed that I work in three hour cycles. If I can get to sleep at midnight, I will, though I may wake at three. If I can't sleep at three, I'm generally up til six. The only real rule of thumb is that I sleep from 6 am to 8 am.
Some day, internet advertising will reach the point where they recognise such trends and target tylenol pm ads at night owls. Har!