Imagine you've set your alarm to go for an early-morning run. But when it goes off at 6, the embrace of your cosy bed is too enticing and the run doesn't happen.
Even those with the best of intentions often struggle to motivate themselves to exercise.
This can feel like a personal failing. But you may just be prey to humans' evolutionary instinct to be lazy.
At least that's the theory of one Harvard professor, who believes our ancestors exerted so much energy hunting and gathering 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 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.
"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 said. "People are often made to feel bad [for not exercising] and I think that's just as ... [wrong] as shaming people for being overweight."