Ask HN: How much of your workday do you spend re-generating yourself?

I am a relatively junior software developer. Because it is not my first career, and I have done some intense/difficult things in the past, I do not have a problem slogging through an 8 hour day (even working through lunch) -- or more. However, a conversation with another developer recently made me wonder if I am approaching things wrong.

He stressed the need for a fresh mind. Admittedly, although I can always push myself through the day, not all of my hours are created equal. On an average day, I believe my coworker "works" less than I do (for example, measured by time his editor is open). Unfortunately, we can't compare output very well, because he also has more experience than I do, and we have different type of roles. (The fact that he's more experienced than me is why I'm looking into his argument and gathering more info.)

Tactically speaking, I would not call what he does "taking a break". It's like 45 minutes or an hour of rather meaningless stuff (YouTube videos, etc.), and sometimes he involves others (easy to do in our open floor-plan office).

So my question is, if you are a knowledge worker, how much of your time at work do you spend regenerating/renewing yourself?

Is the fastest way to progress in my career to do what I'm doing, or what my coworker does? That seems like a simple question but at least part of advancing is appearances (which admittedly don't pertain to actual capabilities in many cases). Is it better to be pushing my brain in the way that I'm doing, or to just operate from a fresh brain all the time by taking many breaks like my coworker?

Enlighten me.