[Question] How long can people be productive in [time period]?

I of­ten see statis­tics that “peo­ple are only pro­duc­tive four hours/​day”, but they never seem to cite their sources. The stud­ies I’ve tracked down are low qual­ity (ex­am­ple). 30 min­utes pok­ing around google scholar pro­duced noth­ing use­ful.

Where are the high qual­ity stud­ies show­ing how long peo­ple can work pro­duc­tivity?