"Is there a healthy and acceptable level of work? According to US researcher Alex Soojung-Kim Pang, most modern employees are productive for about four hours a day: the rest is padding and huge amounts of worry."
Accurate. I max out at 6 hours of productivity. After that, it's diminishing returns, bugs, and bad code.
Although if I'm being honest, that decline probably comes after 4 hours.
https://www.theguardian.com/lifeandstyle/2018/jan/15/is-28-hours-ideal-working-week-for-healthy-life