Exercise boosts your mood, but not everyone has experienced the runner's high, that feel-good rush many exercise fans swear by. New research has found, however, that we are all naturally wired to experience a runner's high when we exercise.

Researchers found that unlike other animals, humans have evolved to experience an overload of a release of pleasurable chemicals in the brain, called endocanabinoids, after an intense workout. That means we're motivated to exercise, and stick to a workout routine, since we're rewarded with feel-good chemicals when we do (it doesn't stop at just a runner's high; a different study found that some women even orgasm while they exercise).

If you're just starting a fitness regimen it may take awhile for your body to experience the runner's high reward, since it takes intense exercise to release the chemicals, researchers say. But don't be discouraged; if you stick with it and keep ramping up your workout, that feel-good rush is just around the corner.

If warmer temps and sunny skies inspire you to hit the woods for your first Spring trail run, there are definitely some obvious dangers to avoid, like bears. You wouldn't think of it, but leaves can also be hazardous, and here's how.

• Wet leaves: When snow melts and Spring rains pour down, you're looking at dangerously wet and muddy trails. The top layer of leaves can appear dry, but underneath, they're a wet, slippery mess, putting you at risk for a nasty fall. Avoid running after a rainstorm and over low areas of the trail that are covered in leaves, since these tend to trap puddles. Wear waterproof trail runners with grippy soles for maximum stability on wet surfaces.
• Leaves covering the trail: In the beginning of Spring, leaf-covered trails make it hard to see what's hidden underneath. Aside from concealing wetness, leaves also cover up fallen limbs, holes, and rocks. Steer clear of running through big piles of leaves, or those that look abnormally high, and try to step on clear spots in the trail.
• Lack of leaves: All the leaves are on the trail, which means there are none on the trees. Bare branches and limbs mean no protection from the sun, putting you at risk for sunburns and heat exhaustion (on really hot days). Aim to run in the early morning or late afternoon when the sun is weakest, and don't forget to wear sunscreen and a hat. It's a smart idea to carry water with you as well to prevent dehydration.

Reese Witherspoon's been having a good week — not only does she turn 36 today, but Reese is reportedly pregnant with her third child. How does the busy actress, mom, and Avon breast cancer ambassador stay fit? Find out Reese's healthy philosophy below.

She's an outdoor runner: Hollywood fitness fads come and go, but Reese sticks to what she knows and loves. Whether it's with her husband or on her own, you can normally find her jogging around town (or going on a hike). The avid runner is onto something; outdoor running helps increase your endurance and strength, since you have to deal with uneven surfaces and the elements. No wonder Reese's legs are so toned!

She sculpts with yoga: Regular runners can benefit from yoga, since the practice can help stretch out a runner's overworked leg muscles and tight hips. Reese practices yoga three times a week, sometimes with her yoga trainer, who encourages her to develop her upper body to balance out her strong lower body. Find out how Reese sculpts her upper body with yoga, with this yoga arm workout.

She's not a dieter: Reese no doubt has been eating even more healthfully now that she's possibly pregnant, but no matter what, she doesn't subscribe to quick-fix dieting schemes. For Reese, exercising and eating right is a way of life. "I work out all the time anyway just to stay healthy. I don't believe in crash dieting or anything like that," she recently told Us Weekly.