Maybe you can work longer and better if you can "cool" your brain.
The brain uses a lot energy, so drink regularly, water if possible, when you learn or think a lot. You can eat something as well.
I don't know about studies, but you seem to know it yourself: when you feel you're not productive anymore, take a break of 5 to 10 minutes.
You could more around, outside if possible, do some gym, because sitting a long time is not confortable.
Refresh your face with water or jump in the swimming-pool if you have one.
Lie down somewhere, close your eyes, and take a 1 minute nap. Breathe slowly and listen to your breath.
Be sure to keep a good mood to avoid headaches, by watching a humoristic video or whatever.
The following is a little off-topic.
I don't know how you learn, but reading alone is generally not effective on the long term. Many studies proved that.
Don't read a book straight, split each chapter, or page, or paragraph, depending on how much there is to learn.
Write the important things you read in your own way, work around it to be sure you understand, repeat it.
If you're trying to learn about one subject for, say, 4 hours, not necessarily straight, and feel exhausted, try to learn something else.
You can put more subjects.