I am a night owl because the evening is the best time to pursue things of personal interest, and when I am on to something really interesting, sleep seems boring and useless in comparison.

Unless I know I have to get up by X hour tomorrow for something important, I will then proceed to stay up until I can't perform physically (e.g. I can't solder because my hands are shaking, or I can't read because my eyelids weigh 50lbs each)