This question, as is, does not particularly describe a well-defined hypothetical.

Am I paying people a higher rate for more hours? The change in productivity might be a net positive or negative if wages rise, depending on the time allocation of the worker. Where is the change in working hours happening? If I go from 1 minute of work to 1 hour of work, productivity would presumably increase because of the time it takes to get into a state of mind to work.

These kinds of things that are needed to answer your question is related to the **time allocation of labor, leisure, and household work** as posited by [Gary Becker in 1965][1]. It has been revisited again by [Gronau, 1977][2] and so on. If I am increasing working hours, I have to decrease activity in household production and leisure, so in order to say whether productivity increases if working hours increase, you have to answer what causes the marginal value of work to increase so that people willingly take on more work and less leisure.

If something like a technological advancement makes it easier for me to work, I may become more productive, but willingly take on less hours to do my work if I am on a salary/commission. This does mean the inverse, that more hours would make me less productive per se, would be true. If I am on a hourly rate, technology that makes it easier to work changes my marginal cost of effort, which changes the opportunity cost of leisure or household work. So maybe I'll take on more work in this case and become more productive.

*The question isn't whether higher working hours decreases productivity. The question is why workers change working hours, and whether those voluntary changes overlap with productivity changes.*


  [1]: http://www.jstor.org/stable/2228949
  [2]: http://agecon2.tamu.edu/people/faculty/capps-oral/agec%20635/Readings/Leisure,%20Home%20Production,%20and%20Work%20the%20Theory%20of%20the%20Allocation%20of%20Time%20Revisited.pdf