We’ve all done it: Resolving to get in shape, we embark on an exciting, new exercise routine—100 pushups a day! Five miles every morning! Ninety minutes of yoga! Then, within a week or two, our enthusiasm peters out, and we’re left with little to show for our efforts.

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Even less ambitious programs often fail because they’re missing a key ingredient that wise athletes and coaches have employed for ages: rest.

Science is clear on this: Your workouts are way more effective when you follow them up with adequate rest.

What Does “Rest” Mean? 

“Rest” has two meanings here, both of which are vital: 1) Time away from intense activity, and 2) Quality sleep. These recovery times allow muscles to repair the micro-tears that occur during exercise, making them stronger than they were before. In fact, your strength gains actually develop not in the gym, but during this recovery period.

Rest times may also lead to denser bones, according to a Canadian study, which found that athletes who skipped rest days had protein profiles that blocked bone formation. Forgo rest time, and your sweat may all be for naught. Plus, rest can prevent overuse injuries if you do the same exercises a lot.

Taking a day off is good for the psyche, too. It averts burnout and boredom and enables you to return to your workout with gusto.

How Much Rest Do You Need? 

The broad rule is: The harder you work, the more rest you need. Studies show that, after a strength-training session, the muscle-recovery process takes about 48 to 72 hours, depending on how taxing the exercise was. So, if you do an intense full-body workout on a Wednesday afternoon, plan to either rest entirely or do just light exercise, like walking or yoga, until at least Friday afternoon. But if you worked only one muscle group, say legs, you could do an upper-body workout on Thursday.

Then, it’ll do your body good to rest entirely at least one day each week. As any religious scholar knows, wisdom doesn’t get any more ancient than that.

Kellee Katagi is one of those strange souls who actually enjoys working out for the sake of working out. She’s spent most of her 20-plus-year writing and editing career covering fitness, nutrition and travel, as well as outdoor sports ranging from skiing to spelunking to street luge (yes, that’s a thing).