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Fight or flight

The stress response has evolved over a millennium to protect you from danger. Also known as the flight-or-fight response, it gets the body ready for action. So if you're in danger, the brain's hypothalamus sends triggers—both chemical and along the nerves—to the adrenals, which are glands that sit on top of each kidney like a hat perched on a head.

The adrenals then churn out hormones, such as cortisol, which raise blood pressure and blood sugar (among other things). This is dandy if you need to outrun a hungry lion, less so if the perceived threat is a looming layoff. And it can be harmful to health if sustained over time.

Next: Cravings