The 40h a week is pretty artificial default that has been imposed on most of the people by business interests. That's how I view it historically at least.
So we might not agree on slippery terms like "basic human rights" but modern phenomena as burnout and work depression are pretty widespread already and can thus serve as an example as to how the current workload standards might be broken.
What's more likely however is that 40h a week seemed more reasonable than the baseline at the time.
Still doesn't make it good for the humans in general though. Time and again studies show humans have no flat productivity rate, and some studies are trying to prove that 40h a week is too much and the people don't have enough productive hours left outside of that to actually tend to family, house cleaning, cooking, leisure hobbies etc.
"There's no such things as "human rights standards" for how much you should work per week."
A 40 hour work week isn't (in my view) a "human right" in the way that life, liberty and property are rights. It's an exchange of time/work for money. People can choose to work a lot or work a little depending on how they value their time vs their leisure.