When you ask your employer for permission for a second job (insofar as that is strictly legal), it's usually not a given that they ask for an estimate of the hours per week you will put in it. Instead they will ask you about the work to see if it might infringe on their business. It's a measure made to ensure that you don't get into their water, not one to protect you. That is why it's in your work contract and not part of the actual law.

In the US, often employment agreements come with clauses that prevent working other jobs, and sometimes even preventing any out-of-work activities that negatively affect job performance.

It’s not just a question of fatigue but also that for intellectually innovative work, it’s rare to get more out of 40+ hour weeks than 20-30.

At least this is the case with kids.

I'm from France, it is not allowed to work more than 44h per week for a period of 12 weeks. But honestly it is difficult to set the same bar for everyone

that is bellow the minimum in japan.

I'm in the UK and have never heard of this 'rule'?