It's just simple physics - the Conservation of Energy - but after you, the designer, have expended a lot of energy - mental, emotional, physical - in getting a design built and finished, after you've crunched right up to your deadline or Opening... you must rest. Maybe you crash.
I'm pretty steady in temperament, even-tempered and cheerful most of the time, but I feel my energy and mood dip after the push to finish a big project. I might be tired and crabby for a couple days.
In olden days (like the 1880s of In the Next Room) I'd have declared myself a bit hysterical and have wrangled myself a nice "cure" at a scenic spa or an amusing Chautauqua... or, if I couldn't get that, at least I'd have had a nice alcoholic swig of Mrs. Lydia Pinkum's Tonic or popped a tasty Liver Pill.
So that famous "artistic temperament"? The manic-depression (nowadays called bi-polar) nature of many artists? I get it. It's perfectly logical. A little extreme, maybe, but perfectly logical.