For me its 9-10pm every night. Its only very few times that I ever go beyond that number. The only time I've ever stayed up until 4am is when I had to stay up that late to take pictures of the night sky with an astronomical research telescope for a class. I hate going to bed so late because the next day I barely have any focus to do anything. Then again, I don't drink coffee, but usually when I get up at 6am I'm fine without it.
10 to half 10, give or take a little. I'm up around 5am most days and I start to get tired around then anyway.
I usually go to bed around 120am, I like to stay up a bit longer and sleep more int he morning, I have a schedule that allows me to do this so no harm in it atm
By 11PM nowadays, even on weekends. Just don't have much interest in staying up too late anymore since the person I'm currently seeing is six hours ahead of me so I'd not be able to talk to them the longer I stay up. Nice feeling not staying up to midnight anymore too. =p
I end up staying up pretty late, but wake up early anyways. As of right now, it's just past 1am and I have to get up at about 5
