Earth & Sky : Transcript - Power Naps: "DB: This is Earth and Sky. A listener writes, 'I was talking to my teacher about 'power naps' and he said that if you sleep for longer than 20 minutes and less than 40 minutes, you get a lot of energy.'

JB: Your teacher's advice is right on. No matter how long you sleep at night, your body's programmed to get sleepy in the early afternoon. A short nap can give you alertness and energy. Studies suggest that a 30-minute nap is usually more restorative than a nap of, say, two to three hours.

DB: That's partly because with a long nap, your sleep can progress into deeper slow-wave sleep, which contributes to 'sleep inertia' -- that grogginess you feel for a while after waking up. Even a short nap can produce some sleep inertia, but it doesn't last long. You can overcome it by increasing metabolic activity in your body and brain -- that is, get up, move around, splash water on your face.

JB: In the early '90s, NASA researchers studied airline pilots on long transpacific flights. They allowed one of three pilots to take up to a 40-minute nap during non-critical times in the flight. The pilots who were allowed to rest were more alert and performed better during descent and landing than the unrested pilots. These so-called 'power naps' can help anyone boost their brain function. That's especially true for healthy people who aren't getting enough sleep. But even though they're great energizers -- naps can't substitute for longer periods of healthy sleep.

JB: That's our show. We're Block and Byrd for Earth and Sky."