How to stay productive while you are studying math? [closed]

Not sure that this question is a good fit for this site, but I will try.

When I am working through a chapter of a mathematical book first two hours are normally very productive (easily remember definitions, can proof many theorems of the text myself), but then productivity goes down even if I eat something sweet and make small pauses.

So after 4 hours my head feels like a slow computer and it takes a lot of time to find a proof even for a simple lemma. The productivity goes back only after an extended break 2-3 hours.

How to extend the productive period, maybe a diet, physical exercises or other tricks you can recommend?

Take a long walk if possible.

I learnt that it helped Andrew Wiles while proving Fermat's last theorem. Hopefully it'll help you too.