I'm looking to understand the inverse relationship between working more hours and less productivity a little bit deeper than just the pop science version

I've heard for years now that evidence shows the more hours you work the less productive you are. I mean anecdotally that makes total sense to me I work two jobs and I'm exhausted all the goddamn time. I recently came across something that said that Japanese productivity was massively low which kind of surprised me I guess just based on my preconceived notions but makes sense in this model. I'm not sure I have a super specific question just trying to understand this particular phenomena a little more.