Wake up in the middle of night and can’t fall back to sleep.

It really depends how long you can sleep before that.

3-4 hours before waking early? You might've woken up at the end of a sleep cycle, which is what people actually aim for when they want a polyphasic sleep pattern. If that happens to me, I'll often lean into it and get up, stop by the bathroom if I need to, grab a glass of water to take back with me, and make sure what I need in the morning is where I should've left it. Usually that means I'll hit the hay again in an hour or two and try to grab that second half. Sometimes I have work in another few hours and I probably just have to live with it for the day.

1-2 hours? Try to pin down what might be waking you up early. A glass of water before bed can actually be an effective alarm clock, so try to avoid that but still pay attention to your thirst and just throw out everything I said here if you think you need it anyway. Noise can also wake us up early, and you might not even know that was it if it ended before you got your wits about you. I also sometimes get woken up by minor earthquakes, and I usually don't know until later because I'm probably too out of it to tell if the floor is jelly or not.

Rather than suggesting taking something since that was like half of the thread already, I'll say another possible cause could be that you're eating too close to bedtime. For me, anything but a really light meal (hard to call a cup of yogurt a meal, but more than that) gets me going again if I eat too late. We're all at least a little different there, but for me I aim for dinner about two hours before bed, and maybe 3-4 hours if there's any slowly-digesting stuff like meat in that meal.