The first thing you should know is, feeling sleepy in the afternoon is normal. It doesn’t mean you had a big lunch, or that you’re depressed, or you’re not getting enough exercise. That’s just how animals’ cycles work — every 24 hours, we have two periods of intense sleepiness. One is typically in the wee hours of the night, from about 2am to 4am, and the other is around 10 hours later, between 1pm and 3pm. If you’re a night owl and wake up later in the morning, that afternoon sleepiness may come later; if you’re an early bird, it may come earlier. But it happens to everyone; we’re physiologically hardwired to nap.
Naps provide different benefits depending on how long they are. A short nap of even 20 minutes will enhance alertness and concentration, mood and coordination. A nap of 90 minutes will get you into slow wave and REM sleep, which enhances creativity. If you sleep deeply and uninterruptedly the whole time, you’ll go through a full 90-minute sleep cycle, and recoup sleep you might not have gotten the night before (we’ve all heard it a million times, but most of us don’t get enough sleep at night).
Try not to sleep longer than 45 minutes but less than 90 minutes; then you’ll wake up in the middle of a slow-wave cycle, and be groggy. I used to hate taking naps during the day for just this reason — I would always wake up in a fog. My problem was I hadn’t yet perfected the art of the 20-minute catnap.
Find a nice dark place where you can lie down. It takes about 50% longer to fall asleep sitting up (this is why red eye flights usually live up to their name), and be armed with a blanket; you don’t want to be chilly. You also don’t want to be too warm, which can lead to oversleeping. (There was a kind of urban legend circulating when I was a kid: don’t fall asleep in the sun, or you’ll never wake up. Not true — but you might wake up three hours later with a ripe sunburn.)
White noise can help you fall asleep, especially during the day when construction crews, garbage trucks, barking dogs and other noisy awake-world things can conspire to destroy your nap. Keep a fan on, or turn on a nearby faucet for a pleasing rushing-river sound. (Just kidding about that last one.)
Don’t nap too close to bedtime, or you might not be able to fall asleep later. Remember, your inbuilt sleepy window is sometime in the early to mid-afternoon — try to nap then.
Quit that silly job where they don’t let you take naps during the day.
Adapted from http://www.mentalfloss.com/blogs/archives/21376