I don't choose to do it, I just can't sleep. I'm sure it's not insomnia, because that's a serious problem and it's more immaturity on my part, but I find it pretty hard to sleep when I try.
Although, strangely, I know what you mean about being able to think clearer. I like to think I've written my best work at night but I don't stay up at night to do it, I just get caught up when I have an idea and waste another hour or so.