I recently read Deep Work which if you haven’t read it yet, I highly recommend. The core point of the book is maintaining a deep level of focus for a long period of time. Where the book falls short is in its explanation into the amount of time needed not being highly focused. The book mentions it briefly but doesn’t dwell on it.
After a period of time I noticed that without breaks my brain would naturally wander off and I needed some time doing so called shallow work to recharge. Turns out that makes sense.