10pm-2am, generally, although right now I'm so exhausted by having to get up at 6 in the morning that I don't think my brain works well at any time. As I age it feels like my body clock is getting to its peak earlier, though.
Bear in mind, since I know a lot of people posting here are teenagers, that teen brains (through to the early 20s) naturally have melatonin kick in late, so staying awake into the night is common for all teens, conversely operating in the early morning is next to impossible for most teenagers. That's why school starts early in the morning. It's the time teenagers are least likely to be able to learn what's put in front of them. I mean, there must be a reason it's then, right?
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"You're never more alone than when you're alone in a crowd"
-Captain Sheridan, Babylon 5
Music of the Moment: Radiohead - In Rainbows