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Why do I feel groggier after a nap?

Do you mean that lumbering where-am-I feeling? It’s a state called sleep inertia, and it occurs when you aren’t strategic about the length of your siesta. See, as you sleep, you cycle through various light, deep, and rapid eye-movement (REM) stages that correspond to different levels of brain activity. If you wake up from a nap in a light-sleep stage, you’ll feel relatively refreshed and alert. But open your eyes near or during a deep-sleep stage and you’ll struggle with sleep inertia.

“The worst impairment occurs in the first 10 minutes after you wake up,” says Kenneth Wright, PhD, an assistant professor of integrative physiology and neuroscience at the University of Colorado at Boulder. Since each cycle lasts approximately 90 minutes, limit your naps to between 10 and 20 minutes to ensure your brain awakens well within that initial light-sleep stage.