Keep putting off that run? You may just be giving in to humans' evolutionary instinct to be lazy.

Imagine you've set your alarm to go for an early-morning run before work. But when it goes off at 6, the embrace of your warm, cozy bed is too enticing and you never make it out the door. Or perhaps you've packed exercise clothes with plans to hit a spin class after work. But then the allure of happy hour, or even your couch, persuades you to skip it.

Even those with the best of intentions often struggle to motivate themselves to work out. There's almost always a powerful temptation to do something, or anything, else.

This can feel like a personal failing, as though the decision not to exercise were a sign of weak character, or at least willpower.

But you may just be giving in to humans' evolutionary instinct to be lazy.

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At least that's the theory of one Harvard professor who believes our ancestors exerted so much energy hunting and gathering that they sought rest whenever they could. We are predisposed to want to conserve energy.

Daniel Lieberman, an expert in human evolutionary biology, posed in a 2015 paper, "Is Exercise Really Medicine? An Evolutionary Perspective," that it's not our natural inclination to exercise for health alone.

"It is natural and normal to be physically lazy," he writes. "I predict that hunter-gatherers in the Kalahari or the Amazon are just as likely as 21st century Americans to instinctually avoid unnecessary exertion. Although a small percentage of people today exercise as a form of medicine, doing their prescribed dose, the vast majority of people today behave just as their ancestors by exercising only when it is fun (as a form of play) or when necessary."

Lieberman explains that our ancestors struggled to amass enough food to make up for the calories they burned tracking down that food. So they needed to conserve their energy when they could. Most modern humans who do exercise don't need to worry about whether after a hard workout they will be able to make up for the calorie deficit.

"Our instincts are always to save energy. For most of human evolution that didn't matter because if you wanted to put dinner on the table you had to work really hard," Lieberman said in an interview. "It's only recently, we have machines and technology to make our lives easier... We've inherited these ancient instincts, but we've created this dream world and the result is inactivity."

He points to escalators in a mall or a subway station. When they are positioned near stairs, most people will choose the ones that move for them. This is often true for elevators in buildings as well. People will drive around a parking lot several times looking for the closest spot rather than park farther away and have to walk the relatively short distance.

Bradley Cardinal, a professor at Oregon State University with an expertise in psychosocial and sociocultural aspects of health and physical activity, isn't entirely convinced that humans' reluctance to move is all biological, although he said he is intrigued by Lieberman's theory.

"I'm still trying to decide if it's learned or biological," he said. "These classic questions of nature and nurture, when I think about that, well, we have a lot of competing things in our environment that make it so hard to move."

Cardinal believes there are factors from childhood that may determine how active you are later in life. In a 2013 study, he found that people who had a negative experience with physical activity when they were younger, such as being picked last for a team, tended to exercise less than those who didn't have that experience.

Still, Cardinal, as well as Lieberman, think this idea that our penchant for rest is an evolutionary trait humans must fight continually may actually improve people's relationships with exercise. There is so much self and societal shame associated with not working out, that going to the gym, or for a run, can often feel like a chore. If they can stop berating themselves and accept that wanting to skip a workout is a completely normal human response, they might begin to untangle the negative associations with exercise.

"People are often made to feel bad [for not exercising] and I think that's just as pernicious and wrong and irresponsible as shaming people for being overweight," Lieberman said. It's not our fault that we are physically inactive, we live in a world that encourages that. They shouldn't be made to feel bad. We need help and we've created a world where we don't have to do it anymore."

Educating people on the health benefits of exercise and movement is not enough to override this basic instinct, he said.

There needs to be a cultural and environmental shift in how our physical activity is prioritised in schools and work. There need to be incentives to move. And although we're not going to return to chasing our dinner, there might be monetary incentives through health insurance, for example, that could encourage people off the couch, he said. He is also an advocate for making physical education a required course in college.

There's also the element of play. One way adults have always done this is through dance. But after their 20s, how many people go out dancing anymore? People are more inclined to be physically active when it's done socially. Lieberman said it's no coincidence that some of the biggest fundraising events in the world are marathons.

"We tie physical activity to community," he said. "I think that has ancient and deep roots. It's a communal issue, so we have to help each other. This individualist attitude we take, it doesn't work that way, it never worked that way. The most effective exercise programs are ones that are social and communal and it's always been that way. People for millions of years went out [hunting and gathering] in groups."