The Netherlands is very Capitalist in my experience and yet they do not have a long hours work culture, whilst for example my own country (Portugal) after the Revolution that overthrew Fascism when the country was pretty Leftwing, had a long hours culture.

Also you see the different takes on working long hours in situations where people are their own bosses and aren't exactly the victims of oppression by their employer.

I think we need to separate people's tendency for disorganised work (which includes a management culture which is bad at organising) which then leads to people working extra hours and the exploitation of people around working extra hours (such as them having no choice but doing it and even them not getting paid overtime).

The former is, IMHO, inherently cultural, but the latter is fully Capitalism in action (and very much reflects the inbalance of power between the Owner Class and the Worker Class when Organised Work is weak and governments do not represent the interests of the majority).

Whilst the two sides are correlated (i.e. if overwork costs money to employees it naturaly tends to be done less than when it doesn't cost extra), it's my impression that in Industries were overwork is paid, and similarly, in two different countries, there is more overwork in countries which don't have a culture of being organised than in those which do have such a culture, even though in both there is a similar pressure not to do overwork because it costs extra money over just having people do the work in their normal work time. I think this is because in the most disorganised countries resourcing, planning and predictability are worse so situations were overwork is the only viable option arise more often even when there are actual higher costs in using it than in doing things during the normal work period.

In summary, I think you are right in that Capitalism is part of the reason, I just think it's not the only reason and the way it interacts with the other causes of the problem is complex, so in some situations Capitalism is very straightforwardly by far the main reason of long hours of work, whilst in other situations it's not at all the reason.

My experience in my own area (software development) is that above working 8h/day people quick start getting chronically tired and tired people make more bugs and more incorrect design decisions, which have to be fixed which is something that consumes far more work than doing things right in the first place - in other words, tired people, alongside doing the work they're supposed to do, also unwittingly create additional work that needs to be done. As the hours worked per day goes up, this effect quickly eats up the gains from working more hours a day and eventually you're actually producing less in overall than you would working fewer hours a day.

Whilst were the sweet spot is varies from person to person, in my experience in my area 10h/day is too much for just about everybody and 13h/day would be insane.

On the other directing you get a reverse effect were you lose work done from fewer hours being worked but you gain some per-hour productivity from being rested that increases work being done. Clearly at 0h/day overall productivity is zero so there must be a point somewhere in there were the losses from fewer hours worked exceed the gains from higher productivity per-hour. Further, I have the impression that productivity gains from being rested actually plateau - as in, you can't really get more rested and productive than a certain level.

As with the other one, I also think it varies from person to person, but I'm less experienced with working shorter hours than working longer hours so don't really know were the sweet spot would be.

Keep in mind that all this is for long term practicing of a schedule, not for, say, people being very tired from working long hours and then switching to working short hours to rest. Recovery from overwork is a whole different ball game and in my experience the fastest way to recover and get back to maximum rest and hence productivity is to just take one (or more) whole days off work, rather than reducing hours worked per day.