I read an article a few days ago which was showing the results of a poll of about 1400 people regarding their productivity times, and it concluded that precisely at 10:04 PM is the best time to get your thoughts straight, while 4:33 PM you probably should avoid doing anything seriously  :D. You can also effectively use bathroom for increasing the chances of getting an inspiration or solving a problem.

For me personally, all these findings works as well, though my night work times are a bit shifted towards later times at night. Anyway, an interesting article and if you would like to read it, here is the link.