[arink]

Rephrasing a bit:

  Most people I know get more done when they aren't interrupted

I'm a morning person, but unfortunately it is much easier to get an uninterrupted block later in the day (6pm and after) than it is early in the morning (6am onward).

If I could function better late at night, I'd switch my schedule in a heartbeat.

[japhyr]

My favorite time of day is 4am. No one else is up, and it's early enough to stay that way for 2-4 hours.

I get foggy and uninspired after 11pm, but in the morning I can fix the world.

[fian]

Uninterrupted time is definitely a factor.

The other factor is when you are supporting a deployed application, you will receive bug reports that require "urgent" attention so end up working late to identify it and either resolve it or detail a workaround until a permanent solution can be found.

Also there is an expectation that IT people will keep up with advances in technology, so many of us spend time at night researching and testing stuff out to keep abreast.