Probably because you're awake playing Starcraft or doing an all night coding session.

Ok, maybe that's just me, but I suspect I can't be the only HN reader who uses his home PC at 3 AM more than he does at 3 PM.

You shouldn't have to turn it off because atleast on Windows 7 - by default it will go to sleep after certain inactivity. And then wake up after a few hours and go to hibernate. Since hibernate is 0 power state, it is as good as turn off. Additionally, you don't lose any of your in-memory data.