Do you get tired in the afternoon? Experts say that's normal. The body has a natural rhythm that includes two drowsy periods every 24 hours, one at night and another between about 2 and 4 p.m. So why not take a nap? Experts say that taking a nap for less than 30 minutes — a so-called "power nap" — allows the brain to rest in a way that makes you more alert when you wake up. Research has also shown other benefits. "We know it's an easy, helpful way to quickly boost your alertness, your concentration, your productivity, your creativity and your mood." says James Maas, who spent 48 years studying sleep as a Cornell University professor. However, people shouldn't nap for too long, as they will fall into a deeper sleep, which is harder to wake up from. Shorter naps are helpful for alertness and memory, says Sara Mednick, a sleep researcher at University of California-Irvine. Her research has found that naps work better than caffeine to improve brain function. For a good nap, find a quiet, dark and cool space and set an alarm for 15 or 20 minutes. Even if you feel like you haven't slept, you still get benefits, Mednick said. People often don't feel like they are asleep until they have reached a deeper sleep. Short naps don't interfere with nighttime sleep, as long as they're not too late in the day. Try to take a nap at the same time every day, around 6 or 7 hours after waking up. The only people who should avoid napping are those who have trouble sleeping at night, Maas says. Older people should be careful because they tend to nap for too long during the day. And if you don't like napping? Research has shown that napping is not for everyone. And Mednick's advice for people who really don't like to nap is: "Listen to your body."