When was a mathematician, I worked as hard as I could until I was mentally exhausted. I actually calculated how much I worked per week over a period of two years. On actual "hard thinking", like working through logic, I spent about 7 hours. There was another 7 hours writing and clarifying topics, and another 6 hours or so on classes, seminars (sort of like meetings, but productive). That gives a total of 20 hours per…
The complicated things are stressing and you need to relax a bit afterwards, and then you can take easy stuff, like maybe an evaluation of what one might do to speed up some process without actually writing the code, just a report. Or simple fixing of design issues, code cleanup stuff like that.