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Why you're tired: You've been sitting all day, every day for months.

What to try: Invite a friend to join you for a brisk walk that lasts at least 30 minutes.

Why it could help: No surprise —people who work out regularly report feeling more energetic than those who don't, research shows. We don't really know the specifics of why, O'Connor says, but it could be a combination of the hits of energy they get from each bike ride, stair climb or hustle across town, as well as the deeper, more restful sleep they earn as a result of having used their muscles. Walking is often recommended as the gateway drug of exercise because it has the lowest risk of injury and sudden cardiac arrest —yet it’s still considered moderate-intensity if you stride with purpose. What's more, sports psychologists have found that one of the most common reasons people (especially women) quit working out is that they felt like they weren't doing the exercises well enough to make a difference. But walking is a beautifully egalitarian exercise that has the power to make anyone feel accomplished.