The Wall Street Journal summarizes the benefits of napping and enumerates the time you should nap for:

For a quick boost of alertness, experts say a 10-to-20-minute power nap is adequate for getting back to work in a pinch.

For cognitive memory processing, however, a 60-minute nap may do more good, Dr. Mednick said. Including slow-wave sleep helps with remembering facts, places and faces. The downside: some grogginess upon waking.

Finally, the 90-minute nap will likely involve a full cycle of sleep, which aids creativity and emotional and procedural memory, such as learning how to ride a bike. Waking up after REM sleep usually means a minimal amount of sleep inertia, Dr. Mednick said.

My favorite part of the article was this tip about taking a 10 to 15 minute nap by holding a pen/pencil in your hand:

Jonathan Brandl is a Newton, Mass.-based consultant who works from home. Up at 5 a.m. to hit the gym, he finds himself fading around 2 p.m. His solution is a fast snooze in a comfy chair in his den. His trick for waking up: He holds a pen or pencil in his hand, which usually falls about 10 to 15 minutes into his nap, waking him up.

Sign me up.