Not only does exercise help you fit into your favorite jeans and maintain your physical fitness, it also improves your mood, decreases stress, and makes you smarter and more creative. In short, breaking a sweat is essentially a wonder drug for your mental and physical health.

But you probably knew that already.

What baffles a lot of people isn't the question of why you should hit the gym, but the puzzle of why, knowing everything you do about the importance of staying active, exercising is still so hard. Despite our good intentions and a flood of public health warnings, a great many of us still struggle to get off our butts regularly.

Laziness is a feature, not a bug.

If that sounds like you, you might find a recent Harvard Magazine article by Jonathan Shaw comforting. It profiles the work of Harvard evolutionary biologist Daniel Lieberman, and apparently according to his trailblazing research, your couch potato ways are not entirely your fault -- humans are actually hard-wired by evolution to tend toward laziness.

In a world where food was scarce, popping out for a recreational jog would be maladaptive -- a waste of precious calories. Our ancestors, therefore, evolved to rest as much as possible in order to conserve energy. They exercised far more than us chasing game and escaping predators, but only because they had to.

"No hunter-gatherer goes out for a jog, just for the sake of it, I can tell you from personal experience," Lieberman tells Shaw. "They go out to forage, they go out to work, but anything else would be unwise."

Sitting around will still make you sick.

That drive to conserve energy made obvious sense for millions of years of human history, but in the modern world, where it's entirely possible to more than adequately feed yourself without getting off the couch, it's a health disaster. And not just because laziness can make you fat.

According to Lieberman, the human body is designed on the assumption that exercise is inescapable and periods of sloth are rare and worrying. In essence, your body shuts down in key ways when you don't move -- muscles waste away, bone repair slows-- in order to conserve energy. These mechanisms for lowering the energy bill of your body made sense at a time when the most likely reason you weren't moving much was because there was nothing to eat. But times have changed dramatically, with worrying consequences, Shaw explains:

... at no prior point in human history was it feasible to lead an existence devoid of activity; exercise was literally part of the environment. The result is that mechanisms for reducing energy expenditure in the absence of physical activity now manifest as diseases. Heart disease, diabetes, osteoporosis, and other maladies of modern life are the consequence of adaptations that evolved as a means of trimming energy demand, and modern medicine is stuck with treating the symptoms.

The bottom line: you can't be entirely blamed for finding it incredibly difficult to get off the couch -- you're programmed by evolution to run only when a lion is chasing you and we live in a lion-less world-- but you really, really need to find a way to make yourself do it anyway.

Does Lieberman have any suggestions? Only that physical education should once again be required at Harvard (and presumably all other schools too), but that won't help those of us who are well past graduation much.

What's your best trick for getting yourself to exercise?