A five-minute nap barely increased alertness, but naps of 10, 20 and 30 minutes all improved the students’ scores. But volunteers that napped 20 or 30 minutes had to wait half an hour or more for their sleep inertia to wear off before regaining full alertness, whereas 10-minute naps immediately enhanced performance just as much as the longer naps without any grogginess.
Why Your Brain Needs More Downtime: Scientific American
A 10-12 min nap satisfies, renews, and doesn’t leave you groggy. Try it! It is one of my productivity secrets.