You can't do the smart work more than 4 hours a day, right, maybe it's just beyond energy exchange capacity in our brain that support extra concentration.
But everyone have a lot of a dumb work, documentation, settling professional conflicts in mail, pinging standstill tasks, routine refactorings or other forms of small product polishing, whatever.
I'm not sure that the 4 of "smart work" is including all those 20% of work giving 80% of value, its sounds funny but sometimes you have too much energy to do very valuable routine.
If you have predisposition to depression (as i do) you can liberate your own time and just waste it to extension your depression experience. Wow it's really worth it.
Depression is just negation of any actions. When external factors is punching your arse its hard to negate. When not, welcome to slow way down.