[Phileosopher]

A huge part of this is tied to the "conscientiousness" aspect of the Big 5 personality.

For myself, I need 1.5-2 days of off-time, depending what I'm working on. I consider myself a constant grindstone-lover, and most of the people I work around seem to reflect your 3.5 number, but I've seen some people who probably need 5 days off a week.

No shame in any of it, though. Everyone's wired differently. The USA has a unique taboo about "enjoying life", and I blame its post-WWII militarized culture for the most of it.