I’m in the fourth decade of my career and I switched to a three day week almost a year ago, taking a proportional pay cut. I work Monday, Tuesday and Wednesday to get the largest contiguous block of time (Thursdays and Fridays) where the places I want to go to are family/child free. If work needs me to swap a day (e.g. to go to a Thursday meeting) I’ll often do that and occasionally I'll work longer one week and then…
It's cut my unexpected PTO to zero, since I can just make up a bad day with a day I'd normally have off.
I try to use my off days to work on some side projects when I'm feeling well.
So far it's been working extremely well. I like working every other day.
Healthier: Yes, it's much easier to manage my condition this way.
Sleep more: Eh, I mostly get up at the same time I'd get up for work on my off days.
Less of a jerk: More like less of a frazzled mess, honestly.
Better at my job: My output is about the same as it was when I was trying (and failing) at working a 5-day week, so the company's getting the same output for less money. I'm generally more relaxed and in less pain, which helps immensely when working with code.