The majority of mortals complain bitterly of the spitefulness of Nature, because we are born for a brief span of life […]. It is not that we have a short space of time, but that we waste much of it. Life is long enough, and it has been given in sufficiently generous measure to allow the accomplishment of the very greatest things if the whole of it is well invested.
— Seneca the Younger, On the Brevity of Life
“I don’t have time” – That’s probably the most common excuse for not exercising. Sure, getting on the treadmill eats up time – time you might use to program, read a book, work on some projects, etc. But what if your mind was clearer, more efficient after you hit the gym? You’d be more productive – maybe so much more productive that the time at the gym would pay for itself.
In this article, I highlight some of the effects of aerobic exercise on the brain, and make an argument that exercise, up to very high intensity – the equivalent of an hour of jogging every day – pays for itself in increased productivity. In particular, you’ll learn how:
- exercise – in particular aerobic exercise – improves you memory by promoting neurogenesis in the hippocampus
- exercise improves executive control, focus and mood by promoting better cerebral blood flow
- this improvement is very substantial – especially in older people
- the time you spend exercising is more than repaid in increased productivity from better focus, decision making, mood, and memory
- exercise will also increase your free time by increasing your longevity
- the effect on longevity saturates at the equivalent of an hour of jogging per day, 6 days a week
Exercise enhances memory & cognition
I’m a big fan of the Learning How to Learn class on Coursera. It introduces laymen to research on efficient and durable learning methods. Hosted by Barbara Oakley – a prof at UCSD who wrote an excellent book on the subject – and Terrance Sejnowski – probably one of the most influential computational neuroscientists around -, don’t let its cheesy production values fool you – it’s legit, and I highly recommend it.
The course covers more than just study habits. In one lecture, we see Terry jogging on the beach in La Jolla, telling us about how exercise can improve your memory. In grad school, I thought of exercise as a time sink; something that pulled me away from doing some real science. But Dr. Sejnowski – who has certainly had a productive career in science – has a point – he was one of the authors of a famous paper on how running enhances hippocampal neurogenesis in rats. Let’s dig into this.
Adult neurogenesis in the hippocampus
The adult brain undergoes neurogenesis during adulthood, and this neurogenesis appears to be limited to two areas: the olfactory bulb and the hippocampus. In the hippocampus, an area associated with the formation of new declarative memories, a few hundred neurons are born every day.
This is true both in animals like rats, which can be studied invasively, and in humans, where a deviously clever observational study has established the turnover rate of neurons in the dentate gyrus of the hippocampus. The concentration of radioactive carbon in the air – and consequently, in the food chain – increased dramatically with tests of the atomic bomb in the 50’s, and fell equally dramatically after the Limited Test Ban Treaty in 1963. By looking at ratios of carbon isotopes in the dentate gyrus of deceased patients born at different times, scientists found that about 700 new neurons are born in the dentate gyrus and integrated into the neural network every day.
A few hundred neurons a day doesn’t sound like much, but these new neurons are very important for the creation of new memories. Neurogenesis can be paused experimentally by injecting a toxin that specifically prevents cells from dividing. Under this manipulation, rats learn much less efficiently a delayed association between a neutral stimulus and an aversive stimulus – classical trace conditioning.
The number of new neurons that get integrated into the neural network is tightly regulated. This number is determined both by proliferation – how many divisions stem cells undergo – and survival – how many of the new neurons are prevented from undergoing programmed cell death.
It turns out that subjecting an animal to a classical conditioning protocol increases the survival rate of neurons. That is, having things to remember causes the very substrate of memory – new neurons – to proliferate. Furthermore, more neurons survive in rats that are reared in enhanced environments with more opportunities for exploration and social interaction. And in black-capped chickadees – a type of bird – hippocampal neurogenesis peaks around the fall, a time of the year where they spend an awful lot of times hiding seeds in caches, and therefore need to form new memories.
It’s as though hippocampal neurogenesis is under a use-it-or-lose-it regime: there’s a homeostatic pressure that prevents the birth and survival of new neurons, unless they’re actually being used.
Neurogenesis IS regulated by exercise
Voluntary exercise promotes hippocampal neurogenesis. Sticking a running wheel in a rat cage increased the number of new hippocampal neurons 3-fold in one experiment. Exercise influences how neurons flourish by many different pathways, most of which promote stem cell proliferation.
One of the main ways by which it acts is by increasing blood flow through the cerebral vasculature. This has wide-ranging consequences beyond just the hippocampus, including higher glucose use and hormones being diffused more efficiently.
Another is by directly promoting the release of various neurotrophic factors, hormones which promote the proliferation and survival of neurons. BDNF, in particular, increases in relation to the amount of voluntary exercise a rat performs.
In a seminal study, van Praag and colleagues – including Dr. Sejnowski, from the introduction – showed that long-term potentiation is enhanced in the dentate gyrus in rats who exercise. Following a rapid sequence of pulses to pre-synaptic neurons, post-synaptic responses were larger in neurons of the dentate gyrus of exercising rats compared control rats. This enhancement in synaptic memory might be driven both by the new neurons integrated into the network, and by the enhancement of memory in older neurons.
Thus, exercise promotes the proliferation of stem cells by enhancing blood flow, regulating the release of trophic factors, and creating an environment where old neurons are more labile.
Cognitive benefits of exercise in humans
For most of our evolutionary history, exertion and having to remember things went hand in hand: migrating to a new area, learning the routes of passing herds for a hunt, remembering the predators which must be evaded. We can speculate that upregulating memory following exercise was adaptive at some point during our evolutionary past. Are the effects of exercise on cognition large enough to matter in the modern world?
Most of the research on the cognitive effects of exercise in humans has been done in older adults, and the evidence for the benefits of exercise there is solid. Many, many different experiments have been done on exercise training in older adults. In general, one group of subjects in the 60-85 age range is assigned to aerobic exercise, while the other is assigned to business-as-usual or a passive treatment, like stretching. Several months later, subjects are assessed on a battery of cognitive tests, sometimes coupled with fMRI studies to assess effects on brain anatomy.
Erickson et al. (2011) show that older adults show a significant increase in spatial memory with exercise. Exercise was accompanied by an increase in serum levels of BDNF – a neurotrophin which mediates stem cell proliferation in the hippocampus, as discussed in the last section. The net effect was an increase in hippocampal volume by about 2%, equivalent to “effectively reversing age-related loss in volume by 1 to 2 years”.
The effects don’t end there. Some of the largest documented effects of exercise are on executive function, which doesn’t depend on the hippocampus, but rather on the prefrontal cortex. Executive function includes working memory, reasoning, inhibition of action, planning, execution, willpower, task switching, etc.
Meta-analysis shows an effect size of .7 standard deviation (SD) units on tests of executive function. To put this into more intuitive numbers, the standard deviation of height in male adults is about 7 cm – for IQ, it’s about 15 points. Imagine if you had an intervention that would raise the mean IQ of a population by 10 points, or mean height by 2 inches – that would be a pretty good intervention!
The wide ranging effects of exercise – better circulation and a better balance of hormones – are probably responsible for this effect on executive function. In fact, although the effects of exercise on the hippocampus are by the far the best studied in animal models, it may that effects outside of the hippocampus are more important – especially in older adults. Voss et al. (2013) point out that the effects of exercise on the brain are broad and not task-specific – in contrast to cognitive training, which tends to be highly task specific.
Thus, older adults are better off spending their time exercising than working on so-called brain games.
Saving time by spending time
It’s clear that whatever the cognitive effects of exercise, it’s not entirely lost time, because exercise has positive effects on general health. To get back to our original question: can you save time by exercising? Perhaps having a more efficient brain can end up saving you enough time – remembering things more easily means you can spend less time studying, looking up documentation, and re-reading things you’ve already read – that exercise pays for itself.
The best evidence for this comes from studies of exercise programs in primary schools and high school. In treatment schools, some time which is usually reserved for instruction – say, an hour every day – is instead used for physical education classes. This is a very direct test of the time saving theory, because instruction time is cut down to make time for physical activity. Results vary from study to study, but meta-analysis shows that kids in treatment schools indeed do a bit better on academic tests – the mean effect size is about .3, which is nothing to sneeze at. That means that spending time on physical activity does indeed more than pay for itself in cognitive benefits alone.
Of course, kids are at a different developmental stage from young adults, and an awful lot of their time is spent learning new things. Perhaps these findings don’t generalize to people like me, young(ish) adults. Unfortunately, there’s little research on how exercise affects cognitive function in healthy young adults.
There have been some studies about the acute effects of exercise on cognition. One conclusion is that moderate exercise increases alertness, which in turn increases performance on a variety of tasks. Certainly, doing some calisthenics in the morning will remove quite a bit of that brain fog you might experience otherwise. However, exercising so much that it causes dehydration hurts cognitive function, which is not very surprising.
So there’s some evidence that the immediate effect of moderate exercise is positive in young adults, but the strength of the effect, and how it and long-term fitness affects productivity is poorly understood. I was surprised, in particular, by the lack of studies on the efficiency of large corporate fitness programs. There are some studies which indicate that real estate brokers and hospital managers enjoy higher productivity when assigned to aerobic classes. In addition, personal fitness habits in small business owners positively influence endpoints like sales and productivity. However, much of this data comes from self-reports and the sample sizes are quite small.
This gap in the literature is quite surprising, given how much people complain about cognitive overload… you would think that somebody would have crunched the numbers. Given that big, data-driven corporations continue to implement corporate fitness programs, however, the net impact on productivity is likely to be positive; we just don’t know how much.
How much should you exercise?
So how much should you exercise? We can try a microeconomics approach and maximize the total utility of our time, measured in productivity years in adulthood:
So far, we’ve been discussing the increase in productivity that comes from better cognition and memory, but any analysis of the value of exercise also has to take into account its effect on longevity, and how much time it eats up.
The latest research indicates that exercise increases longevity by a wide margin; compared to completely sedentary people, people who exercise can decrease their mortality risk by 40%, equivalent to increasing their life expectancy by 5 years.
The largest observed effects are at quite high levels of activity, however – at around 40 metabolic equivalent hours (MEH) per week. MEH is a measure of activity intensity in terms of energy spent compared to being at rest. That comes out to about 6 hours of jogging or 15 hours of brisk walking a week (!).
That much exercise will eat up significant chunks of time. if you do 6 hours of jogging a week for your entire adult life, your life expectancy will increase by 5 years – but you’ll spend more than 2 years of your life jogging! Perhaps it would be better to spend a more modest 2 hours of jogging a week – that still increases your life expectancy by 4 years over being sedentary, and you’ll spend less than a year in your life exercising.
The perfect amount of exercise depends on how it affects the productivity multiplier in the equation above. Unfortunately, we don’t know the dose-effect curve for productivity vs. exercise in healthy adults.
The school physical activity studies referred to earlier do show however, that the multiplier is sufficiently high that it more than compensates for the time taken by the exercise. This is at quite high levels of activity – we’re talking an hour a day of vigorous activity on every school day. It’s still not quite the data we’re looking for – but it does indicate if you spend two years of your life jogging, it may not be a timesink.
Exercise promotes brain health by improving blood flow, which helps deliver glucose, and promoting a better balance of hormones and trophins. In the hippocampus of adult mammals, including humans, exercise promotes neurogenesis, with measurable improvement in memory function. Exercise improves executive function, particularly in older adults; it decreases risk of depression; and has acute not fully understood, including an increase in alertness.
All these effects could help you think clearer and live a richer, more productive life. We don’t have a lot of data on how exercise affects productivity, but the research that is there is tantalizing: in school children, in small business owners, and in large corporations, spending time on exercise rather than work ends saving more time than it takes.
The right amount of exercise you need to be your own best self is hard to measure, but recent research on longevity indicates that it could be quite high – in the range of 20 to 60 MEH per week. That comes out to 3 – 9 hours of high-intensity activity or 6 to 20 hours of moderate activity a week. In other words: A LOT!
There’s some obvious gaps in the literature. Here’s a few studies I’d love to see:
- A rigorous study of the dose-effect relationship of aerobic exercise on productivity in young adults, using an objective measure of productivity – e.g. number of keystrokes in office workers, number of code check-ins for programmers, etc.
- A randomized control trial in large corporations that incentivizes people to work out by paying people their regular salary when they’re working out, plus a small bonus
- A study of the acute effects of different exercise regimens on cognitive; i.e. jogging vs. strength training vs. yoga, etc.
In the meantime: get out there and play, and don’t forget to hydrate!
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