Like you, probably, I have found myself writing endlessly into the night on more than a few occasions. External contingencies like exams and work deadlines are usually the reason, but sometimes internal motivation has driven me to keep my eyes open for a bit longer as well. Here is what I've found helps me to get the most out of this kind of situation.

Use body clock to take small breaks

And what helps is first of all to avoid what doesn't help. I have a high tolerance for discomfort, or maybe I just have an exceptionally obsessive mind, but many times I have continued working without break long past the point where I am cold, thirsty, hungry, needing to pee or sore in my eyes, neck, shoulder or forearm.

What I have learned is this: Take care of your basic physical needs (besides sleep) when writing into the night for a more comfortable experience. As soon as you need to pee, use it as a quick break. Obsessive mind reaction: No, WAIT! MUST. FINISH. THIS. PARAGRAPH. Basic physiologic needs responds: Yes, you must - in two minutes. Following the FIRST impulse is important. Follow the body clock. Otherwise you will begin a mental endurance feat of some length and intensity, which is besides the point of your immediate task. Practice the ability to suspend, to bear, to feel, to accept, the intense wish to FINISH things NOW.

Wake up your mind with movement

At least every hour or two, take a short break to do gentle movement, bouncing, shoulder and arm movements, or a breath exercise like airless squats to wake up your brain.

Don't crash with heavy food or sugary snacks

Eat light snacks and drink tea or coffee substitutes like four sigmatic mushroom extract. Avoid heavy food. A good snack is carrots with humus.