I cannot take breaks. I have to make 100% of the time productive.

What I've accepted is that you can't work on the same thing 100% of the time, or to work with 100% intensity.

I have my 5-6 hours of intensive work (divided between morning and afternoon, coinciding with my peak hours), and the rest of the time I devote to low-intensity work like reading on Medium, watching video-essays...

And that low-intensity blocks feed the ideas that I'll work on during the high-intensity blocks.

So I'd say the key is not to save energy, but to diversify it.