Napping is a great practice for long and harried days. Even a quick dip into Light Sleep or REM can have a profound effect on our energy levels, as well as our emotions and performance.  A 1994 study by NASA found that productivity increased noticeably, sometimes as much as 34% for certain tasks, after a nap of less than 30 minutes.

That said, you don’t need to have a long nap to reap its benefits. Two separate studies published in the journal Sleep found that a 10 minute nap produces more renewed vigor and better cognitive performance than a 30 minute nap. 

In fact, those who napped only 10 minutes had no signs of impaired alertness and performance upon waking, while those who slept 30 minutes did show signs of poorer performance and alertness right after awakening.

Why?

Longer naps may slip into Deep sleep, which can result in us feeling more sluggish than before. This effect is called sleep inertia and it can impair our abilities in pretty noticable ways. For instance, driving while experiencing sleep inertia can be as dangerous as driving drunk.  Those who had shorter naps most likely did not go into Deep sleep, thereby giving them the quick recharge without the sometimes negative side effects.

So if you’re going to catch a quick forty winks, best to keep it cat-nap sized.