What is the most effective maximum work duration per day?

Question

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.

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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.

Answer 1

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 & 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 & Longman (1978) and Dvorak et al. (1936), and studies on other perceptual motor skills (Henshaw & 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 & 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., & 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., & Ford, G. C. (1936). Typewriting behavior. New York: American Book Company.
Ericsson, K. A., Krampe, R. T., & 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., & 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., & Schlosberg, H. (1954). Experimental psychology. New York: Holt, Rinehart & Winston.}

Answer 2

It would appear that a 6 hour workday would obtain better results of productivity than a >=8 hour day.

In discussing the optimum length of a working day, I have taken a neurobiological approach and an occupational approach. As the individual is not an isolated organism; social and environmental demands, mean that in determining what is a most productive workday, needs to encompass the fact that the individual has other tasks to complete within a 24 hour cycle. The need for sleep, to maximise performance, also needs to be taken into consideration, within this context.

There are been much research showing the relationship between excessive work hours, resulting fatigue and decline in cognitive ability. The effect is also magnified as the working week progresses. Following this, there is evidence that a reduced working day (from 8 hour) increases productivity:

Working time and productivity International Labour Office Geneva

The Effects of Working Time on Productivity and Firm Performance: a research synthesis paper; has examined this subject extensively:

a. Reduction in hours and worker productivity evidence
The potential theoretical and practical impact of a reduction in hours on productivity was assessed by the International Labour Organization (ILO) over twenty years ago (White, 1987). Improvements in the efficiency of labour utilization were evident from a century’s worth of research that found some productivity improvement following a reduction in hours, depending, of course, on the accompanying conditions and responses, in the medium if not the short term. Four types of reductions were distinguished, all of which remain relevant to today’s conditions. Each creates its own potential for productivity improvements that would offset much, if not all, of the initial costs associated with shorter working hours. The four types are: reductions in excessive hours, gradual reductions in standard hours, accelerated reductions in standard hours, and individualized options for reducing working hours. ...

a. Interactions between duration and employee-centered flexibility of work schedules
Importantly, the effect of working time flexibility often interacts with the duration of working hours. Greater discretion or control over the timing of their work helps workers to alleviate some of the negative effects of long hours on the incidence of work-related injuries, illnesses and time stress (Boden, 2005; Costa et al., 2006; Dembe et al., 2007; Hughes and Parkes, 2007). ...
Reduced variability of hours has almost as much influence as higher flexibility on work/life satisfaction (Costa et al., 2006).

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There has also been research into the best time of day to work. Research suggests that the peak times for alertness are mid to late morning (approx 11am), followed by another, lessor, peak in the late afternoon (approx 5.30pm).

Circadian Rhythms.

Human beings have natural Ultradian cycles, that govern our behaviour. I am focusing on the circadian rhythm, which is the natural 24 hour cycle that governs our physiology. It would be logical to propose that working within this cycle, as opposed to enforcing routines that are in conflict with this cycle, would be beneficial to finding an optimal working day structure.

Wakefulness
The longer the brain has been awake, the greater the spontaneous firing rates of cerebral cortex neurons with this increase being reversed by sleep. Another effect of wakefulness (which may or may not be related to this) is that it lowers the small stores of glycogen held in the astrocytes that can supply energy to the brain's neurons—one of the functions of sleep, it has been proposed, is to create the opportunity for them to be replenished.

Sleepiness The longer the time spent awake, the greater the drive for sleep. ..... The second factor is circadian, which varies with a 24 hour periodicity and is independent of the amount of preceding sleep or wakefulness. .... Together, the homeostatic and circadian factors modulate the need for sleep and influence the balance between alertness and sleepiness. ....Neurobiology of sleep and wakefulness Robert W. McCarley and Christopher M Sinton (2008) doi:10.4249/scholarpedia.3313

The following charts provide a good visual representation of the bodies 24 hour cycle:

The pink line is the predicted alertness
The blue dots are recall data

chart taken from the followup to "Polyphasic Sleep: Facts and Myths" Dr Piotr Wozniak

image courtesy of File:Biological clock human.svg

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• I disagree with rethinking the smaller 90 minute work and break cycles. My explanation for this is given clearly in this answer. There is evidence, even within a shorter workday, that the 90 minute cycle exists and breaks enhance performance. I am going to focus on the optimal total hours of a working day.
• There are various physical, emotional, mental, financial and environmental factors (eg job stability) that would cause a variation in what an optimal working day would be; so for the purposes of this question I am assuming we are discussing individuals, within the norms for these factors (ie not unwell in these areas or experiencing undue stress).
• I am assuming that you want the focus of this answer to be on work, rather than sleep. So, without going into discussion about optimal sleep patterns; it follows, logically, that a regular, extended working day of, for example: 16 hours, would prohibit the individual from getting optimal sleep, which in turn affects performance. So, for the sake of this discussion, I am assuming that an optimal work day, takes into account the need to optimize sleep, and that the individual is doing so.
• I'm not addressing shift work here; as the scope becomes too large.
• The relationship between optimizing such activity as for example: playing the violin, needs to take into account the effect on the fingers; whereas in office work, there is less propensity for such risk of repetitive strain or injury.
• There has been research regarding compressed working weeks, the benefits of completely 5 days work on 4 days.

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Full-time vs Part-Time work:

The following provides evidence for and against job performance and comparisons between full-time and part-time work hours. It is interesting to note that the difference in these findings could be related to the reward system of the job. The differences in performance, could (and probably are) be linked to the capacity to increase earnings with improved performance vs a fixed wage rate.

Looking at this alone, it would suggest that shorter working hours promote better performance when financial rewards are offered for higher productivity. Which supports the notion of a shorter working day being more optimal for productivity.

Argument for Part-time over Full-time work:
(full-time work is classified as a 40 hour working week)

Part-time workers tend to be more productive, hour for hour, than full-time workers because it is easier to work at peak efficiency for short periods.

An interesting, if not obvious point, made by Berkley University HR. Flexible Work Arrangements: Part-Time Work

A comparison of part-time vs. full-time salespeople from four U.S. direct selling companies that part-timers had greater job satisfaction and less propensity to quit. Part-timers were also better performers as measured by earnings per hour worked.

Another interesting finding in this study, opens a whole avenue for analysis, between having options and motivation upon performance.

When these same respondents were analyzed in terms of other jobs held simultaneously with their direct selling job, some evidence indicated that job satisfaction was lower while earnings per hour and propensity to quit were greater as the extent of other outside employment increased.

Full-time vs. part-time salespeople: A comparison on job satisfaction, performance, and turnover in direct selling Thomas R. Wotruba doi.org/10.1016/0167-8116(90)90014-E

Argument for Full-time over Part-time work:

A interesting note with regard to the differences of work outcomes between full-time and part-time workers; the only notable difference is regard to job, the implication being that full-time workers are more likely to have a more responsible attitude towards their jobs. If the overall working week were to change, this difference would need to be re-examined.

Employees who work fewer hours per week tend to assign less importance to the work outcomes measured in this study.

Differences in the importance of work outcomes between full-time and part-time hospital employees Douglas S. Wakefield1, James P. Curry1, Charles W. Mueller2, James L. Price2 DOI: 10.1002/job.403008010

In studies on employee attitude, their was no difference between full-time and part-time workers.

A meta-analysis was conducted (k =38, N =51,231) to examine the size of the difference between full- and part-time employees on job attitudes. Results indicated that there was little difference between full-time (FT) and part-time (PT) employees on job satisfaction, organizational commitment, intentions to leave and facets of job satisfaction.Full-time employees were found to be more involved with their jobs than PT employees.

Job attitudes of part-time vs. full-time workers: A meta-analytic review Todd J. Thorsteinson DOI: 10.1348/0963179037659136

In terms of performing tasks beyond immediate job criteria, the results sway towards full-time employees rating better, but this is also not clearly defined. This could be linked towards employment attitude.

There are two types of performance: in-role and extra-role work performance or OCB ... Marchese and Ryan (2001) found significant differences between full-time and part-time workers in job performance. Full-time employees had a higher level of performance. Another study showed no significant difference between full-time and part-time workers in terms of performance (Wotruba, 1990)

A Comparison Between Full and Part-Time Lodging Employees on Job Satisfaction, Organizational Commitment, and Job Performance Abdullah Al Omar, Jet al

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Shorter workdays:

There is clear evidence that a reduction of workday to 6 hours benefits work performance. This could be due to many complex, pyscho-social factors.

There are some company-based interventions that have studied the effects of a reduction in workhours from 7 or more to 6 hours. In an intervention study among female health care workers, a decrease in workhours (to a 6-hour workday) resulted in improvements in the social life of the workers and in moderate improvements in well-being when the group was compared with a reference group with no changes in workhours. In another study, a shift to 6-hour workdays was followed by a reduction of neck-shoulder and back pain in three separate organizations when fulltime payment of the workers was retained. According to Anttila, the benefits of shorter workhours were the most apparent in regard to social life, but they also introduced some positive effects on the perceived stress of the workers ...

Scand J Work Environ Health 2006;32(6, special issue):502-514 Workhours in relation to work stress, recovery and health by Mikko Harma, MD

Physical effect of shorter workday:

The shortening of regular workdays from >7 hours to 6 hours may considerably reduce the prevalence of neck-shoulder pain among persons with physically demanding care work. The potential health benefits should encourage intervention studies also in other occupations with increased risk of work-related musculoskeletal disorders.

A shorter workday as a means of reducing the occurrence of musculoskeletal disorders by Ebba L Wergeland, PhD, et al Scand J Work Environ Health 2003;29(1):27-34

As @Jens noted in Ericsson's study.

best students practiced on average 3.5 hr per day

This, along with the evidence about attention and circadian rhythms, perhaps, suggests that this is an optimal time for mental acuity. (not taking into consideration the physical restraints of finger strain on violinists).

The double peak of alertness within the circadian cycle gives credence to the notion of a "siesta" of prolonged, daily work break, to boost productivity, but this argument is not as straightforward as it may seem on the surface.

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Studies which may be helpful:

This study 15 Indus. & Lab. Rel. Rev. 307 (1961-1962) Shorter Hours - In Theory and Practice; Dankert, Clyde E. gives detailed analysis, where output optimum is examined and optimum working day and optimum working week evaluated.

Orthostatic symptoms, blood pressure and working postures of factory and service workers over an observed workday Suzy Ngomoa, et al doi.org/10.1016/j.apergo.2007.11.004

I do not have access to these articles

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Associated References:

A META-ANALYSIS OF WORK DEMAND STRESSORS AND JOB PERFORMANCE: EXAMINING MAIN AND MODERATING EFFECTS SIMONA GILBOA1, ARIE SHIROM1, YITZHAK FRIED2, CARY COOPER3 DOI: 10.1111/j.1744-6570.2008.00113.x

Extended workdays: Effects on performance and ratings offatigue and alertness ROGER R. ROSA DANIEL D. WHEELER and JOEL S. WARM MICHAEL J. COLLIGAN

CIRCADIAN RHYTHMS OF PERFORMANCE: NEW TRENDS 2000, Vol. 17, No. 6 , Pages 719-732 Julie Carrier1,2 and Timothy H. Monk3 doi/abs/10.1081/CBI-100102108

Carrier, J., & Monk, T. H. (2000). Circadian rhythms of performance: New trends. Chronobiology International, 17, 719–732. doi:10.1081/CBI-100102108.

Schmidt, C., Collette, C., Cajochen, C., & Peigneux, P. (2007). A time to think: Circadian rhythms in human cognition.Cognitive Neuropsychology,24, 755–789.doi:10.1080/ 02643290701754158

Health effects of shift work and extended hours of work J M Harrington

Adverse changes in mood and cognitive performance of house officers after night duty. D. I. Orton and J. H. Gruzelier

Extended workdays: Effects of 8-hour and 12-hour rotating shift schedules on performance, subjective alertness, sleep patterns, and psychosocial variables DOI:10.1080/02678378908256877 Roger R. Rosaa, Michael J. Colligana & Paul Lewisb

Professor Cary Cooper CBE publications

Changes in Electromyographic Activity Associated with Occupational Stress and Poor Performance in the Workplace Frank E. Gomer Behavioral Science Applications, Yellow Springs, Ohio Louis D. Silverstein Sperry Corporation, Phoenix, Arizona W. Keith Berg University of Florida, Gainesville, Florida Donald L. Lassiter Georgia Institute of Technology, Atlanta, Georgia

Job content and working time: the scope for joint change DOI:10.1080/00140139108967349 KAZUTAKA KOGIa

IS PART-TIME EMPLOYMENT BENEFICIAL FOR FIRM PRODUCTIVITY? Annemarie Nelen, Andries De Gripa, Didier Fouarge

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Note:

There is much anecdotal evidence of professional working 4 or 5 hour days to maximise their performance; one example being an author writing a book.