We define naps as mid-day sleeping of under an hour. Longer sleep periods during the day are called siestas.
Recent research actually shows that midday naps can reduce the risk of cardiovascular disease, especially for males. A report published in the Annals of Internal Medicine reported on a large-scale study of people in Greece. Naps reduced the risk of heart problems about as much as statin drugs do.
A study in 2006 concluded that regular naps of less than 30 minutes can improve productivity and mental performance. Regular longer naps are associated with higher mortality. The authors felt that regular nappers could get the most out of their naps by "training" the body to awaken after a short nap.
People who napped at least 30 minutes a day, three times a week, were 37 percent less likely to die from heart disease. Occasional nappers had a 12 percent reduction. Although there is always a risk that daytime naps lead to nighttime insomnia, individuals can learn the specific needs and response of their bodies. Many people can nap in the daytime without nighttime problems. A study of seniors in a retirement community found no significant impact of napping.
Between 2 p.m. and 4 p.m., your body naturally experiences a small dip in temperature, signaling the brain to produce the sleep-inducing hormone melatonin. Sleep plays an important, if not wholly understood, part in formation of long term memory. Memories are consolidated during sleep. Brain researchers have shown events they call "spindles" happen in Stages 1 and 2 and these seem to be connected with memory formation and learning. Short naps can be very effective in facilitating learning. They are especially effective in consolidating learning of new motor tasks. Thus the "power nap". It is thought that power nap might accelerate memory consolidation by inducing NREM sleep. More here. And here.
One theory is "synaptic pruning" takes place during sleep. This holds that during waking period synapses grow stronger and the number of synapses increases, crowding out the brain's ability to absorb more information. Sleep is a time when the brain eliminates the number of synapses and frees up resources for further learning. Previous learning is sent to long-term memory. This theory has some animal evidence to support it, but it is just a theory.
You might hear about microsleep periods and think these are naps. They are not. Microsleep is unintentional brief (as little as a few seconds) of sleep, often of only part of the brain, and usually unknown (not consciously perceived) by the person. You do not want to have microsleep episodes. Indeed, intentional naps are a preventative measure against microsleep.
Retired people take a lot of naps because they have less structured days than younger people, but those of all ages can take naps.
What used to be called a catnap is now called a power nap. The word "power" makes it acceptable for working adults who think of themselves as on the top of their game and helps sell napping to people who might otherwise think of it as an activity for small children and old people. Calling is a "power nap" makes it socially acceptable.
Usually the power nap is under 20 minutes, so the brain doesn’t have time to go through all the phases of sleep. Longer naps often leave the person groggy upon waking, but power naps can be refreshing without a sleepy hangover.
Some people take their power nap at their place of work - at their desk chair for instance. More ambitious nappers have a cot near their office or even go out to their car for a nap. Most do not use alarm clocks. Trivia: the Prophet Muhammed was a fan of napping and advised people to nap.
The Boston Globe published a one-page guide to napping. It's pretty good although for some reason they include a drawing of a monkey in the tree.
All other things being equal, what is better: an afternoon nap or getting more sleep at night? There is no correct answer of course, but the addition of 30-45 minutes in nighttime sleep does not significantly affect measures of vigilance and daytime sleepiness the next afternoon. Mid-day naps do improve performance on the psychomotor vigilance test and they make people less sleepy in the afternoon as measured by the MSLT. Caffeinated beverages also help us over the mid-afternoon hump more than extra sleep at night, too.
Little kids often nap as part of their regular day. Old people are also stereotypical nappers. In both these cases, age-related sleep patterns can explain part of the predilection. A Pew Reseaech Center survey found that 34% of U.S. adults nap on any given day. Among those past age 80, the percentage was 52%. Men are more like to nap than women and regular exercisers were more likely to nap than sedentary people.
The timing of the nap affects the sleep architecture. Morning naps and afternoon naps differ, with people tending to drift off faster in the afternoon for longer naps with more slow-wave deep sleep than morning naps.
Lifehacker article:The Science of the Perfect Nap