James Maas is king of the "power nap." A former Cornell psychology professor turned corporate sleep guru, Maas coined the phrase. "I invented it while consulting at IBM years ago," he says. "I wish I had trademarked it."
A proper power nap, Maas says, is short--15 to 20 minutes, "long enough to rejuvenate, but not make you groggy or give you insomnia later." Ideally, you should lie down--although it's possible to nap in a chair--close the door, and turn off the lights. Though longer naps are more restorative, they require longer recovery times.
In fact, if you go past 20 minutes, Maas suggests sleeping a full 90 minutes to get a complete REM sleep cycle, rather than rousing yourself from a deep sleep. To decrease grogginess, some experts recommend drinking coffee right before your nap. It takes about 20 minutes to kick in, so you'll wake up with a (caffeinated) jolt of energy.