For an energy boost: Nap just 10 to 20 minutes when you’re seeking a quick burst of energy and alertness. Your brain typically will reach only the lightest stage of sleep, “making it easier to hit the ground running after waking up.”
For cognitive memory processing. Nap a full hour if you want to reach slow-wave sleep, a stage that aids in the remembering of facts, places, and faces. But there is a downside: You’ll tend to feel groggy upon waking once your nap exceeds 20 minutes.
For improved creativity. Naps of 90 minutes to two hours let your brain run through a full sleep cycle, including the rapid-eye-movement stage. Getting through a full cycle eliminates grogginess. Better yet, it “aids creativity and emotional and procedural memory”—good for learning to play the piano or ride a bike.
Source: Wall Street Journal