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Your Brain Might Not Be Cut Out for a 40-Hour Work Week

After age 40, you can only do your best work for 25 hours each week

Are you over 40?

Would you be a more effective employee if you only worked three days a week?

Twenty-five hours.

Maybe Tuesday, Wednesday and Thursday?

Colin McKenzie thinks so.

McKenzie is at the Institute of Applied Economic and Social Research in Melbourne, Australia and professor of economics at Keio University in Tokyo. He and his team have recently conducted two studies — one looking at the impact of work hours on cognitive functioning and another, yet to be published, on how work hours affect health more generally. Both arrived at the same conclusion for men over 40: Your ideal work week is 25 hours long.

The study included a memory test, a pattern-recognition test and a reading/communication test. After 25 hours, overall performance for the test subjects decreased as “fatigue and stress” took effect. McKenzie says 40 is a tipping point because it’s when most people perform less well at such mental agility exercises. Needless to say, his research is more relevant than ever before now that fewer people are saving for retirement and more are working well into their golden years. And so, I recently spoke to him about why the brain seems to turn to shit after 40 and whether this is further ammunition for corporate overlords to go younger and cheaper.

What were the key findings from your research?
We observed that cognitive functions improve when you increase working hours from zero. In all cases, however, the measures seemed to peak around 20 to 24 hours of work per week. After that peak, cognitive functioning starts to decline. At about 45 or 50 hours a week, you reach the level of cognitive functioning as if you weren’t working at all. And if you work more than 50 hours a week, you’re worse off compared to if you weren’t working at all.

Do the cognitive tests in your study mimic things you’d be asked to do at a normal job?
It depends on the job. These are relatively simple tasks. Memory is obviously an important thing for working in any job — as is the ability to communicate and identify similarities and differences between two or more things. The tests, however, are really designed as preliminary tests for identifying dementia.

You’ve been quoted saying that “working hours can be a double-edged sword.” How so?
The standard story in the medical literature about effects of retirement is that stopping stimulation on your brain with work-related activities results in cognitive functioning declining. But if you work too much and are subject to increased stress and fatigue, there’s existing evidence about how that impacts different aspects of health and cognitive function. In other words, there are positive effects from working — stimulating the brain, using muscles in the body and moving about — but too much stimulation leads to fatigue or exhaustion.

And the declines begin at age 40?
As people get older, cognitive functioning is known to decline. We’re not as interested in age-related declines in cognitive functioning. We’re more interested in how changes of hours worked affect cognitive functioning and health outcomes.

Did the same declines apply to younger people?
We didn’t examine people under the age of 40.

Why — especially if you weren’t interested in how age affects cognitive functioning?
We’re interested in the causal impact of working hours on cognitive functioning. In order to be able to make statements about causal impacts, we need to be rule out “reverse causality” — that is, people reducing their working hours because their cognitive functions and/or health status have declined. To do so, we use information about changes in pension eligibility ages in Australia. However, the further people are away from the pension eligibility age, the less likely this information will be informative about changes in working hours. This is critical to being able to make causal statements. Given this, we made the decision to exclude people under 40.

How old are you?
I turn 60 in December.

How much do you work? Do you follow the results of your research?
I don’t.

Why not?
I want a certain lifestyle, and I can’t support it working 20 to 25 hours a week. Lots of people work long hours — either their company says that they’ve got to do it or they like working. We’re trying to make people aware that the effect of working hours on cognitive function is important, but not the only factor to take into account when deciding if you want to work 20, 40 or 60 hours a week.

Did you experience this shift at age 40, where you couldn’t work as effectively after 25 hours in a week?
There are things like memory and your ability to manage certain things that you become more aware of — your ability to learn new things and remember dates and time declines. At the same time, as you age, you gain experience and can learn from experience, so you can manage better.

Is this a progressive phenomenon? Does it get worse at 50 compared to 40?
You need a lot of data from other ages, and we don’t have that yet.

The implications are pretty serious — that older employees might not be able to work as many hours as younger ones. What’s been the reaction to your study?
A big debate occurred in Australia, where there was a political party also promoting a three-day work week. We have no connection/alignment with the party, nor did the debates have any connection with our research — we started it years beforehand. But we got a lot of press in Australia for that reason.

In Japan, where I am now, no one’s picked it up, even though the issue of long work hours is a hot topic here. For instance, one of the major ad agencies has just been prosecuted for overworking its staff — resulting in suicide. But no one cares about our academic research here. They have a good excuse, though: It’s been published in English, and the Japanese press isn’t so good at picking up research that’s not published in Japanese.

Given middle-age men work best at 25 hours, are there things we can do to maximize those times of peak brain power?
We have some hypothesis on things that might be relevant, but we don’t have any scientific evidence yet. We think how people use their time outside their working time could be important. For example, working mothers who go home and do lots of housework may be in a different position to working husbands working the same hours but who do zero housework.

There’s also some existing evidence for a limited range of occupations about the impact of shift work. So whether you’re always working 9 to 5 or your hours of work vary over the week/month could be important. Similarly, if you worked a 24-hour week, it might matter if you only worked eight hours for three days rather than four hours for six days or six hours for four days.

Finally, the pattern of workdays and holidays could be important. In a seven-day work week, are two consecutive holidays better than, say, a holiday on Wednesday and a holiday on Sunday?

Is there anything we can do to combat this phenomenon?
In another paper, we looked at what a person’s “career job” (i.e., the job you spent the most time in) was and found that there are certain job types that have smaller reductions in cognitive function after retirement. For example, people who use math skills in their job are much less likely to suffer cognitive function declines. That’s not exactly what you asked, I know. But job choice is potentially important in the long term in predicting likely cognitive decline.

Is love of the job relevant—doing what you love?
We have some evidence in different research that the characteristics of a person’s career job affect the extent of cognitive decline after retirement. Love of the job may be relevant, but measuring such an extremely subjective factor would be extremely difficult.

Given that it’s already a young person’s work world and employers are always looking for younger, cheaper types, doesn’t this research essentially give them scientific evidence with which to justify this opinion?
Since we don’t know about what happens to young people, I don’t think our research provides any justification for this view. The scientific evidence suggests that cognitive functioning declines with age after some point, but there may be offsetting factors from a work point-of-view — namely, the importance of experience. Personally, I’m happy to compete with new young academics because my experience gives me a lot that they don’t have.

Do you think we’re headed toward a time when more people are working only three or four days a week?
If you take the really long historical perspective, work hours have been in decline for the last 100 to 200 years. How much they decline is dependent on how much money people want to earn and what level of work companies are happy with and how technology impacts on different things. How much 40- to 50-year-old people are going to be working in 10 or 20 years is going to be very much dependent on the job that they’re in. And that makes it a very hard thing to predict.