I find that, for me, this is a symptom of specialization and the “day job”.

If I can dive into some coding one week and whip the garden and landscaping into shape the next and work on a book the next et c, I thrive working longer hours than that.

It’s when I’m doing the same. Damn. Shit. Day after day, week after week, month after month, that I feel drained to exhaustion after doing just a few hours of it.