Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

Like the modern concept of exercising for the sake of health, treadmills are recent inventions whose origins had nothing to do with health and fitness. Treadmill-like devices were first used by the Romans to turn winches and lift heavy objects, and then modified in 1818 by the Victorian inventor William Cubitt to punish prisoners and prevent idleness. For more than a century, English convicts (among them Oscar Wilde) were condemned to trudge for hours a day on enormous steplike treadmills.

Among the many studies of the Hadza, one asked forty-six Hadza adults to wear lightweight heart rate monitors for several days.14 According to these sensors, the average adult Hadza spends a grand total of three hours and forty minutes a day doing light activities and two hours and fourteen minutes a day doing moderate or vigorous activities. Although these few hours of hustling and bustling per day make them about twelve times more active than the average American or European, by no stretch of the imagination could one characterize their workloads as backbreaking. On average, the women walk five miles a day and dig for several hours, whereas the men walk between seven and ten miles a day.15 And when they aren’t being very active, they typically rest or do light work.

The Hadza, moreover, are typical of other hunter-gatherer groups whose physical activity levels have been studied. The anthropologist Richard B. Lee astonished the world in 1979 by documenting that San hunter-gatherers in the Kalahari spend only two to three hours a day foraging for food.16 Lee might have underestimated how much work the San do, but more recent studies of other foraging populations report similarly modest physical activity levels as the Hadza.17 One especially well-studied group is the Tsimane, who fish, hunt, and grow a few crops in the Amazon rain forest. Overall, Tsimane adults are physically active for four to seven hours per day, with men engaging in vigorous tasks like hunting for only about seventy-two minutes a day and women engaging in almost no vigorous activity at all but instead doing mostly light to moderate tasks such as child care and food processing.18

All in all, assuming that what hunter-gatherers do is evolutionarily “normal,” then comprehensive studies of contemporary foraging populations from Africa, Asia, and the Americas indicate that a typical human workday used to be about seven hours, with much of that time spent on light activities and at most an hour of vigorous activity.19 To be sure, there is variation from group to group and from season to season, and there is no such thing as a vacation or retirement, but most hunter-gatherers engage in modest levels of physical effort, much of it accomplished while sitting. How different, then, are such “normal” humans from postindustrial people like me (and perhaps you), not to mention farmers like the Tarahumara, factory workers, and others whose lives have been transformed by civilization?

FAO scientists decided to measure people’s energy expenditures using the simplest metric possible, the physical activity level, or PAL.20 Your PAL is calculated as the ratio of how much energy you spend in a twenty-four-hour period divided by the amount of energy you would use to sustain your body if you never left your bed. This ratio has the advantage of being unbiased by differences in body size. Theoretically, a big person who is very physically active will have the same PAL as a small person who does the same activities.

Although there is much variation, PALs of hunter-gatherers average 1.9 for men and 1.8 for women, slightly below PAL scores for subsistence farmers, which average 2.1 for men and 1.9 for women.21 To put these values into context, hunter-gatherer PALs are about the same as those of factory workers and farmers in the developed world (1.8), and about 15 percent higher than PALs of people with desk jobs in developed countries (1.6). In other words, typical hunter-gatherers are about as physically active as Americans or Europeans who include about an hour of exercise in their daily routine. In case you are wondering, most mammals in the wild have PALs of 3.3 or more, nearly twice as high as hunter-gatherers.22 Thus, comparatively speaking, humans who must hunt and gather all the food they eat and make everything they own by hand are substantially less active than average free-ranging mammals.

The average PAL of industrialized adults in the developed world is 1.67, and many sedentary individuals have even lower PALs.23 These declines, moreover, are relatively recent and largely reflect changes in how we work, especially the growth of desk jobs that glue us to our chairs. In 1960, about half of all jobs in the United States involved at least moderate levels of physical activity, but today less than 20 percent of jobs demand more than light levels of activity, an average reduction of at least a hundred calories per day.24 That modest amount of unspent energy adds up to twenty-six thousand fewer calories spent over the course of a year, enough to run about ten marathons. And outside our jobs, we walk less, drive more, and use countless energy-saving devices from shopping carts to elevators that whittle away, calorie by calorie, at how much physical activity we do.

If you are an average adult American male who weighs 180 pounds (82 kilograms), your rate of energy expenditure while resting quietly in a chair is approximately seventy calories per hour. This is your resting metabolic rate (RMR), so named because your resting metabolism comprises all the chemical reactions going on in your body while you are not being physically active. Based on your RMR, we can calculate that if you do nothing but sit in a chair for the next twenty-four hours, your body will expend about seventeen hundred calories.

Seventeen hundred calories is a lot. Even when you are sitting, you are not entirely at rest. Some of that energy is being spent digesting the last meal you ate, regulating your body temperature, and preventing your body from slumping to the floor.

To correct for these expenses, we could measure your energy expenditure in bed just after you woke up from an eight-hour sleep in a dark 70°F room following a twelve-hour fast. That measurement, your basal metabolic rate (BMR), would be roughly 10 percent lower than your RMR (in our example, 1,530 calories). Your BMR is the energy you use to maintai...

How does the amount of energy you spend at rest compare with your total energy budget? To compute this ratio, we next need to measure your total daily energy expenditure (DEE), the overall number of calories you spend over the course of twenty-four hours doing everyth...

If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories a day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism. Who would have thought that being a couch potato was so expensive?

The key lesson to digest from the starving men’s dramatically lower resting metabolic rates is that human resting metabolisms are flexible. Most critically, resting metabolism is what the body has opted to spend on maintenance, not what it needs to spend.

If you are sitting while reading this, for every five breaths you take, one pays for your brain, another for your liver, a third for your muscles, and the last two pay for the rest of your body.

Starvation also turns out to be rare in preagricultural societies because hunter-gatherers live in tiny populations within enormous territories, they are not dependent on crops that can fail, and when times are tough, they move to find food. Decades of research show that hunter-gatherers generally manage to avoid starvation and maintain about the same weight throughout the year.18 That doesn’t mean hunter-gatherers don’t face tough times. They do. In fact, they frequently complain of being hungry. But one of the ways hunter-gatherers survive is by not foolishly squandering scarce calories on unnecessary activity.

In contrast, a typical hunter-gatherer woman takes about eighteen years to mature into an adult, and when she becomes a mother, she needs at least 2,400 calories a day to take care of her body plus nurse an infant. To get more energy than the chimpanzee mother, the human mother eats a higher-quality, more diverse diet of fruits, nuts, seeds, meat, and leaves, some of which she gathers herself but some of which is provided by her husband, her mother, and others. Further, by cooking her food, she gets more energy than by eating it raw.25 Thanks to that extra energy, instead of having a baby every five or six years, the human mother weans her infants earlier and typically has another baby about every three years. Thus, human mothers typically care for and feed more than one dependent child at a time. In addition, humans take 50 percent longer than chimpanzees to reach maturity, magnifying the time and cost of raising each offspring.

For this admittedly unscientific study, I stood for ten minutes at the bottom of the stairs and counted how many took the escalator versus the stairs. Of the 151 people I observed during those ten minutes, only 11 took the stairs, about 7 percent. Apparently, people who study and promote exercise are no different from the rest of us. Worldwide, the average is just 5 percent.26

High-quality studies that measured thousands of people find that average adult Americans are sedentary 55 to 75 percent of the time they are awake.21 Given that most Americans are in bed about seven hours a night, the average time spent being effectively immobile adds up to between nine and thirteen hours a day. Keep in mind these averages mask considerable variation from person to person and over time. Unsurprisingly, Americans tend to be more active on weekends and become increasingly sedentary with age. Whereas young adults tend to sit about nine to ten hours a day, older individuals average slightly more than twelve hours a day.

Although not all latter-day Americans sit as much as some alarmists suggest, we are more sedentary than earlier generations. There is evidence that the total time Americans spent sitting increased 43 percent between 1965 and 2009, and slightly more for people in England and other postindustrial countries.

By convention, your heart rate during sedentary activities is between its resting level and 40 percent of maximum; light activities such as cooking and slow walking boost your heart rate to between 40 and 54 percent of maximum; moderate activities like rapid walking, yoga, and working in the garden speed your heart rate to 55 to 69 percent of maximum; vigorous activities such as running, jumping jacks, and climbing a mountain demand heart rates of 70 percent or higher.27 Large samples of Americans asked to wear heart rate monitors indicate that a typical adult engages in about five and a half hours of light activity, just twenty minutes of moderate activity, and less than one minute of vigorous activity.28

In contrast, a typical Hadza adult spends nearly four hours doing light activities, two hours doing moderate-intensity activities, and twenty minutes doing vigorous activities.29 Altogether, twenty-first-century Americans elevate their heart rates to moderate levels between half and one-tenth as much as nonindustrial people.

The most egregious claim is that we can efficaciously prevent or treat almost any disease—from autism to Parkinson’s—by simply avoiding “pro-inflammatory” foods like gluten and sugar or by wolfing down “anti-inflammatory” foods like turmeric and garlic. If these miracle diets seem too good to be true, they are.

The bad news is that chronic inflammation plays a role in many serious diseases. The good news is that the biggest causes of chronic inflammation are largely avoidable, preventable, or addressable: smoking, obesity, overconsumption of certain pro-inflammatory foods (a chief one being red meat), and—surprise, surprise—physical inactivity. Which brings us back to the topic of sitting. How would an innocuous few hours relaxing in a chair inflame my body?

In contrast, even squatting or kneeling requires some muscular effort, just standing burns about eight more calories per hour, and light activities like folding laundry can expend as much as a hundred calories per hour more than sitting.36 These calories add up. By merely engaging in low-intensity, “non-exercise” physical activities for five hours a day, I could spend as much energy as if I ran for an hour.

Another worrisome way sitting can provoke inflammation is through psychosocial stress. I hope you are reading these words contentedly on a beach or some other pleasant place and not fretting about sordid things like swollen fat cells and inflammation. Sadly, sitting is not always relaxing. Long hours of commuting, a demanding desk job, being sick or disabled, or otherwise being confined to a chair can be stressful situations that elevate the hormone cortisol. This much-misunderstood hormone doesn’t cause stress but instead is produced when we are stressed, and it evolved to help us cope with threatening situations by making energy available. Cortisol shunts sugar and fats into the bloodstream, it makes us crave sugar-rich and fat-rich foods, and it directs us to store organ fat rather than subcutaneous fat. Short bursts of cortisol are natural and normal, but chronic low levels of cortisol are damaging because they promote obesity and chronic inflammation. Consequently, long hours of stressful sitting while commuting or a high-pressure office job can be a double whammy.

But among the men who were fit, those who sat the most had a 65 percent higher risk of inflammatory-related diseases like type 2 diabetes than those who sat the least. An even larger study based on survey data from more than 240,000 Americans found that time engaged in moderate and vigorous activity lowered but did not erase the risk of dying associated with being sedentary.46 Even those who engaged in more than seven hours per week of moderate or vigorous exercise had a 50 percent higher risk of dying from cardiovascular disease if they otherwise sat a lot. Altogether, these and other alarming studies suggest that even if you are physically active and fit, the more time you spend sitting in a chair, the higher your risk of chronic illnesses linked to inflammation, including some forms of cancer.47 If these results are correct, then exercise alone doesn’t counter all the negative effects of sitting.

A multiyear analysis of almost five thousand Americans found that people who broke up their sitting time with frequent short breaks had up to 25 percent less inflammation than those who rarely rose from their chairs despite sitting the same number of hours.

These activities aren’t serious exercise, but experiments that ask people to interrupt long periods of sitting even briefly—for example, just a hundred seconds every half hour—result in lower levels of sugar, fat, and so-called bad cholesterol in their blood.52 In turn, less circulating blood sugar and fat prevent inflammation as well as obesity. In addition, small and occasional bouts of moving stimulate muscles to quench inflammation and reduce physiological stress.

In a famous 1986 study, Eric Ravussin and colleagues asked 177 people to spend twenty-four hours (one at a time) in an enclosed ten-by-twelve-foot chamber that measured precisely how many calories they spent. To the researchers’ surprise, the individuals who fidgeted spent between one hundred and eight hundred calories more per day than those who sat inertly.

Other studies have found that simply fidgeting while seated can expend as much as twenty calories an hour as well as promote beneficial levels of blood flow to restless arms and legs.56 One study even found a 30 percent lower rate of all-cause mortality among people who fidget after adjusting for other forms of physical activity, smoking, diet, and alcohol consumption.57

Because desk jobs are here to stay for the foreseeable future, standing desks have been widely advertised as a panacea for excess sedentariness. Such marketing deceptively confuses not sitting with physical activity. Standing is not exercise, and as yet no well-designed, careful study has shown that standing desks confer substantial health benefits.

One massive fifteen-year-long study of more than ten thousand Danes found no association between time spent sitting at work and heart disease.60 An even bigger study on sixty-six thousand middle-aged Japanese office workers yielded similar results.61 Instead, leisure-time sitting best predicts mortality, suggesting that socioeconomic status and exercise habits in mornings, evenings, and weekends have important health effects beyond how much one sits during weekdays at the office.62

Sleep is such a vulnerable state that animals like zebras sleep only three or four hours a day because they are in constant fear of lions, whereas lions that eat the zebra typically enjoy about thirteen.

During NREM sleep, specialized cells throughout the brain expand the spaces between neurons by as much as 60 percent, allowing cerebrospinal fluid that bathes the brain to literally flush away this junk.16 These opened spaces also admit enzymes that repair damaged cells and rejuvenate receptors in the brain for neurotransmitters.17 The only catch, however, is that the brain’s interstitial pathways are like single-lane bridges that let cars pass in only one direction at a time. Apparently, we cannot think while cleansing our brains. We thus must sleep to flush out the cobwebs left behind by the day’s experiences.

No one knows exactly when and where that eight-hour prescription originated, but during the late nineteenth century striking factory workers marched through city streets shouting, “Eight hours for work, eight hours for rest, eight hours for what we will!”

New sensor technologies that monitor sleep objectively indicate that the average adult in the United States, Germany, Italy, and Australia tends to sleep about six and a half hours in the summer when it is warm and light and between seven and seven and a half hours in the colder, darker winter months.

The most electrifying study by far was by the UCLA sleep researcher Jerome Siegel and his colleagues, who affixed wearable sensors to ten Hadza hunter-gatherers from Tanzania, thirty San forager-farmers from the Kalahari Desert, and fifty-four hunter-farmers from the Amazon rain forest in Bolivia. None of these populations have electric lights, let alone clocks or internet access. Yet to Siegel’s astonishment, they slept less than industrialized people did. In warmer months, these foragers slept on average 5.7 to 6.5 hours a day, and during colder months they slept on average 6.6 to 7.1 hours a night. In addition, they rarely napped. Studies that monitored Amish farmers who shun electricity as well as other nonindustrial populations such as rural Haitians and subsistence farmers in Madagascar report similar average sleep durations, about 6.5 to 7.0 hours a day.

Yet since then, numerous studies using better data and sophisticated methods to correct for factors like age, illness, and income have confirmed that people who sleep about seven hours tend to live longer than those who sleep more or less.

So if you sometimes wake up in the middle of the night or sleep seven rather than eight hours a night, relax. In fact, humans appear to be adapted to sleep less than our ape relatives, including chimpanzees. This reduction possibly evolved about two million years ago when our ancestors apparently lost many of the features that help us climb trees, which offer a safe place to sleep in the wilds of Africa. As slow, unsteady bipeds who had to sleep on the dangerous ground, we must have been easy pickings for leopards, lions, and saber-toothed tigers before we harnessed fire. Under such conditions, our vulnerable forebears might have gone extinct had they not slept lightly, minimally, and in staggered bouts so that someone in the group was always awake to raise the alarm.

If you are unsure about your own sleep health, sleep researchers suggest you ask yourself five simple questions:71 Are you satisfied with your sleep? Do you stay awake all day without dozing? Are you asleep between 2:00 and 4:00 a.m.? Do you spend less than thirty minutes awake at night? Do you get between six and eight hours of sleep? If your answers to these questions are “usually or always,” then you should sleep contentedly knowing that you generally get enough sleep.