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Why Are Morning Runners So Happy?

09/11/2015 05:04pm ET | Updated September 11, 2016
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By Teal Burrell for Runner's World

If you're a morning runner who is often out the door before you have breakfast, a new study suggests that you might be more likely to experience the runner's high.

The theory revolves around a hormone called leptin. Leptin is linked to feelings of fullness and satiety. It increases after eating, helping to quell hunger. Endurance athletes who exercise regularly tend to have lower leptin levels (if you're training for marathon and are constantly famished, you understand).

In a new study, published in the journal Cell Metabolism, researchers examined the impact of leptin on the brains of mice--as mice have leptin just like humans. Scientists disrupted leptin signaling in one group of mice, tricking their brains into thinking their leptin levels were low. The mice in this group ended up running double the mileage on their running wheels than mice in another group whose leptin levels were unaffected.

Additionally, when given a choice between a chamber with a running wheel in it and one with a locked, unusable wheel, the mice with lower leptin preferred to spend most of their time in the chamber associated with running.

These findings reveal that lower leptin makes mice want to run farther, suggesting they enjoy it more. Stephanie Fulton, a neurobiologist at the University of Montreal and senior author of the study, believes this could also apply to humans. It appears that lowered leptin levels in runners may play a role in the euphoria and enjoyment associated with the runner's high.

It makes sense that regular runners who train consistently, and therefore have lower leptin levels, enjoy the sport and feel happy when out logging miles. It may seem paradoxical, though, that something that causes hunger also makes us want to run more and feel happier while doing so. But the theory makes sense from an evolutionary perspective. When early humans were low on food, they would have to endure long treks to find more. The runner's high might have evolved to motivate them to keep going, to ensure they would get the food they needed.

It's important to recognize that these findings do not mean that you should purposefully run too far or too often on an empty stomach, as you'll likely just bonk--a miserable experience that's far from euphoric. It's important to fuel your body properly so that you can run--and feel--your best.

The discovery simply provides a scientific theory as to why a short early morning run can feel so invigorating.

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