I have my own theory on the why's of this schedule. Many hackers got their start in computers at an early age, while still in school. Therefore, the only time available to hack was in the evening (after school you are hanging with your friends). And once started on an interesting project, it is hard to stop -- hence the 3-am hacking session.
In college, you are often studying late (much more work in college then high school) -- therefore this continues the trend. After college, you are spending your days programming for your boss, working on projects that may not interest you. Therefore, to keep your sanity, you are forced to do your personal projects late at night again. And after all, this is when you are used to hacking on things from years of experience, so your brain thinks it's normal.
In college, you are often studying late (much more work in college then high school) -- therefore this continues the trend. After college, you are spending your days programming for your boss, working on projects that may not interest you. Therefore, to keep your sanity, you are forced to do your personal projects late at night again. And after all, this is when you are used to hacking on things from years of experience, so your brain thinks it's normal.