For me this is the most critical part. It's not enough just not to be interrupted. I have to feel like I couldn't be interrupted.
That's the great thing about programming at night. If everyone else is asleep (except other people who don't want to be interrupted) they won't even think of interrupting you.
Sometimes, I wind up working late on those days because I lose track of time and I actually don't mind. I find zone time related to my personal happiness, if I don't get into the zone, I become short fused and frustrated.
Lucky for me, I'm one of the principals of the company. So I can work till 4am and stroll into the office at noon.
By the way, the music I zone out too is classical. I have playlist of symphonies and after each one I stretch out. when Strauss' "Also sprach Zarathustra, Tod und Verklärung" are (is) over I know it's time to stand up.
Or look at your workspace and figure out why the fuck people need to stay up until 4am to be productive, rather than being able to, you know, work, at work?
As for the 2 programmers who work for me (the rest of the staff have to be there 9 to 5) they can work anywhere anytime. I care about results; meet your schedule- you can work from the beach if you want (I live in South Florida).
There's something very tranquil and calm about knowing the outside world is asleep. The dark has the effect of making it hard to see anything that isn't immediately in front of you, so when you're coding you don't see anything but the screen and the keyboard.
It kind of gives you nothing else to think about other than the problem to be solved.
It reminds me of this talk by Jason Fried, which is so obvious but no conventional workplaces solve these issues: http://www.ted.com/talks/jason_fried_why_work_doesn_t_happen...
http://www.walgreens.com/store/c/mack%27s-ear-seals-dual-pur...
They succeed in disturbing you without actually disturbing you, it's great (for them).
(These jaunts only happen about once a week though, so the output on my hobby projects is pretty weak.)