We’ve all heard the classic saying “work hard, play hard.” But maybe it’s time to add “rest well” to this adage. After all, giving your body and mind time to rest and recover makes them stronger, which enables you to put in more effort in the future.

Just take Deena Kastor, a collegiate runner. She had never won a major race and so, hell-bent on achieving one, she went to train with the legendary coach Joe Vigil. Under his tutelage, she far surpassed her prior levels of success. When asked how she’d done it, she said the significant impact was between training sessions – not the training itself.

More specifically, Kastor gets 10-12 hours of sleep every night, plans her diet out in great detail and receives a weekly massage and stretching session. Such a rigorous recovery program has enabled incredible growth.

So why is rest so crucial?

Well, one reason is that suppressing or resisting needs, rest included, is a stressful mental task that makes all others much more difficult. For instance, in the mid-1990s, the social psychologist Roy Baumeister became interested in why people feel tired after approaching a complex problem and why humans run out of willpower in general.

To study the question, he got 67 adults together in a room that smelled of freshly baked chocolate chip cookies. Then, the cookies themselves were brought into the room. Half of the people in the room were allowed to eat the cookies, while the other half were told to eat radishes instead.

Naturally, for this second group, resisting the cookies was difficult.

Then, after both groups had eaten, they were asked to solve a problem that appeared solvable but which was in fact impossible. The radish-eaters gave it 19 attempts and gave up in eight minutes; meanwhile, the cookie eaters tried 33 times, committing over 20 minutes to the task.

Simply put, the radish eaters had worn out their mental muscles resisting the cookies, while the cookie eaters were still full of energy.