Beauty sleep: There’s a reason the phrase gets dished out in everyday jargon. When you wake from a deep, restful snooze, you may notice your skin looks supple, your eyes look bright, and you sport an allover glow. The opposite, unfortunately, is also true: When you’re up tossing and turning, you may wake up puffy, bleary-eyed, and sallow. But what’s the why behind beauty sleep (or lack thereof)? 

It has everything to do with cortisol.

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