Too much stress and concentration could make it tougher for you to speed up on the road or treadmill

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You know those runs when you just can't seem to get into the zone? It may be fatigue, but not the kind you thought: A new study in the journal Medicine & Science in Sports & Exercise found that when runners were mentally tired (thanks to stress or hours spent concentrating super hard on a work task, for example), it negatively impacted their pace. (Check out these tricks to Shave A Minute Off Your Mile.)

How is a tired mind to blame? When study participants ran while mentally fatigued, their run not only felt harder, but actually was harder: Researchers noticed sluggish-minded runners had lower oxygen consumption (when it comes to VO2 Max, the higher, the better for your fitness). Looking for ways to relax your brain? Check out 8 Secrets Calm People Know.

Exercising first thing in the morning can help since you won't be bogged down by the day's mental burdens and stress. But, if nighttime sweat sessions are more your thing, here's a reason to power through even if you feel psychologically sluggish: Exercise itself is one of the best ways to reduce stress. So while it may be tougher to get through it, you'll feel better afterwards-physically and mentally. For an extra boost, try one of these 5 Moves For Instant Energy before you lace up your sneakers.