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A short nap of around 20 minutes can improve alertness without taking someone too deeply into sleep.
As a nap gets longer, the brain can enter deeper stages of sleep. Waking during deeper sleep can cause sleep inertia—that heavy, confused, “Why am I even more tired?” feeling after waking up.
That's why short naps are commonly recommended when the goal is simply to recharge during the day.
Longer naps aren't automatically bad, but they serve a different purpose and may interfere with nighttime sleep for some people.
Quick takeaway: For a daytime energy boost, keeping a nap short can make waking up easier.
© Sleep Foundation