What is the most effective maximum work duration per day? A related question asks about the best work-break schedule for an effective eight hour work day.

This question presupposes that eight hours of work per day is a good duration and that all you need is to optimize your work-break schedule. But research seems to hint that alertness and productivity might increase if you shorten your workday: A survey (Bielenski, 1994) asking managers about their experiences with part time work in eight European countries found a better quality of products or services. Part time work here includes all different kinds of reduced weekly working hours. A German (Infratest Sozialforschung, 1985) and American (Beyer, 1986) study on flexible work time models (including part time and gliding time) found an increase in production volume and quality, higher job satisfaction, and a decrease in lateness and absences.

Research also shows that performance increases if you increase your sleeping hours (e.g. from eight to ten hours per night), and it is a banal truth that the the amount of sleep that you can or want to allow yourself is directly related to the amount of work: eight hours of work plus taking care of the rest of your life only leave you so many hours to rest. So maybe the question of breaks doesn't even arise anymore, when you awake relaxed and highly attentive after enough sleep, and when you work focussed for four hours without breaks, knowing that you'll be free to enjoy yourself for the rest of the day.

Assuming on the one hand that you do in fact need time to get things done and cannot speed up your work frequency and shorten working hours indefinitely; and assuming on the other hand that you will get tired and work less well after a certain time; then there should be a turning point somewhere, where working longer or shorter decreases your productivity.

Of course the exact location of this point (and the optimal per day work duration) will differ for different kinds of tasks, so there will not be one optimum for everything. If there is more research on this topic than you can or want to cover in your answer, please focus on "intellectual work" (like that of scientists or in administration).

Question:

Is there research into how long you should work per day to be most effective?

I'm looking for a comprehensive and definite answer on this, and therefore I am especially interested in recent meta-analyses and overview articles, some of which might be published in handbooks or encyclopedias of occupational psychology and not available on the web.

No untested theories and no personal anecdotes, please.



Sources:


Beyer, H. T. (1986). Betriebliche Arbeitszeitflexibilisierung. München: Vahlen.


Bielenski, H. (ed.) (1994). New Forms of Work and Activity. Survey of Experiences at Establishment Level in Eight European Countries. Shankill: European Foundation for the Improvement of Living and Working Conditions.
Infratest Sozialforschung (1985). Flexible Arbeitszeiten: Erfahrungen aus der Praxis. Humanisierung des Arbeitslebens, vol. 68. Frankfurt am Main: Campus.
This is a part-answer to your question and it doesn't come from occupational psychology, but from expertise research. 

Findings from Expertise Research

In their seminal paper on the role of deliberate practice for the acquisition of expert performance, Ericsson et al. (1993) report a number of constraints that play a role in the acquisition of expert performance. One of these constraints is the effort constraint. What is meant by this is that, in order to improve one's skill, one has to engage in deliberate practice, which by definition is an effortful activity that requires full concentration. It is clear that such an activity cannot be kept up for very large amounts of time. Ericsson et al. cite works by Welford (1968) and Woodford &amp; Schlosberg (1954) (which I haven't read) that compare the efficiency of practice durations ranging from 1-8 hr a day. These studies find that there is 


  essentially no benefit from durations exceeding 4 hr per day and reduced benefit from practice exceeding 2 hr. (Ericsson et al., 1993, p.370)


The authors then report that studies on the aquisition of typing skill by Baddely &amp; Longman (1978) and Dvorak et al. (1936), and studies on other perceptual motor skills (Henshaw &amp; Holman, 1930) which 


  indicate that the effective duration of deliberate practice is may be closer to 1 hr per day. (Ibid.) 


Finally, they cite a study by Pirelli &amp; J.R. Anderson (1985) that found 


  no increased learning from doubling the number of training trials per session. (Ibid.)


Thereafter, Ericsson et al. (1993) turn to more intellectually demanding domains. They report on data from financially independent writers, who 


  tend to write only 4 hr per day, leaving the rest of the day for rest and recuperation (Cowley, 1959; Plimpton, 1977). (Ibid., p.371) 


In their own study 1, which investigated violinists at music college, the authors found that the best students practiced on average 3.5 hr per day.

Maximum vs. Typical Performance

From these findings it seems that the optimal duration lies somewhere between 1 and 4 hours. But it's important to note that this only holds for conditions of maximal effort and full concentration. Furthermore these studies investigate the acquisition, not the execution of a skill, which presumably is more important in the workplace. I would still argue that they can still be relevant in a work context.

In io-psychology there is the distinction between maximum and typical performance. Everything I have said so far obviously refers to maximum performance. The intellectual work of a scientist that you mention in your question might fall into this category. I can also imagine that a software developer needs full concentration. Probably, though, periods that require maximum performance and those that "only" require typical performance alternate. The time limit for typical performance may be much longer.

So my answer is only a part answer in at least two ways: First it only provides information about the time limit for maximum performance. Second, the studies that I cite have not actually been done within an occupational setting.

References:

Baddeley, A. D., &amp; Longman, D. J. A. (1978). The influence of length and frequency of training session on the rate of learning to type. Ergonomics, 21(8), 627-635.
Cowley, M.(Ed.). (1953). Writers at work: The Paris review interviews, second series. New York: Penguin Books.
Dvorak, A., Merrick, N. L., Dealey, W. L., &amp; Ford, G. C. (1936). Typewriting behavior. New York: American Book Company.
Ericsson, K. A., Krampe, R. T., &amp; Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363–406. doi:10.1037/0033-295X.100.3.363 PDF
Pirolli, P. L., &amp; Anderson, J. R. (1985). The role of practice in fact retrieval. Journal of experimental psychology: Learning, memory, and cognition, 11(1), 136.PDF
Plimpton, G. (Ed.). (1976). Writers at work: The Paris review interviews. New York: Viking Press.
Welford, A.T. (1968). Fundamentals of skill. London: Methuen.
Woodworth, R. S., &amp; Schlosberg, H. (1954). Experimental psychology. New York: Holt, Rinehart &amp; Winston.