sleep
Everyone likes a good nap now and then, right? Whether you nod off during a boring movie, or rest your head on your desk at work for 20 minutes or so to fight the afternoon slump, naps can revitalize you in a major way.

One study even showed they can boost performance and memory regulation better than caffeine. This all sounds great in theory, but many people — myself included — find naps do the opposite.

I wake up from naps feeling like I'm in the throes of a New Year's Day-strength hangover. It takes me at least 20 minutes to recover from them, and I never end up seeing any of the benefits. Even when I timed my nap to be no more than 30 minutes — the nap length sleep experts claim is the most beneficial — I came out of it certain I was experiencing the early stages of the flu (I wasn't).

Naturally, I've always been a little jealous of the people who take naps and wake up feeling like a million bucks. I'm a healthy, youngish, childless woman who regularly sleeps seven to eight hours a night — why don't naps work for me?

The short answer is that some adults are genetically predisposed to need more hours of continuous sleep than others (I'm leaving children out of this because, as growing bodies, they naturally need more sleep). According to a study by the National Heart, Lung and Blood Institute, at least 80 genes appear to be involved in sleep regulation, which "suggests that sleep duration in natural populations can be influenced by a wide variety of biological processes."

Simply put, sleep duration needs vary considerably because they're based on a broad spectrum of genetic differences.

But that's just natural sleep rhythms. When you factor in that 1 in 3 Americans are sleep deprived, it makes sense that roughly the same percentage of Americans take a nap once a day, regardless of the benefits they may or may not reap. It's the body attempting to make up for the imbalance so it can continue performing adequately.

"Sleep is extremely important to our overall well-being, and therefore loss of sleep or poor quality of sleep can significantly impair one's ability to function," says Neomi Shah, associate division chief of Pulmonary, Critical Care and Sleep Medicine at the Icahn School of Medicine at Mount Sinai. "A nap may just be the right thing to overcome any transient perturbations in sleep quantity and quality."

Considering the pervasiveness of sleep deprivation and chronic sleep disorders in America, it's unsurprising that there have been numerous studies published about the potential benefits of napping.

And while a nap can certainly help make up for insufficient sleep, your ability to feel and perform better post-nap hinges on your physiology — how receptive your body is to a less consolidated sleep schedule.

People who average six hours of sleep a night or less, for example, tend to be "habitual nappers" and seem to benefit most from the occasional nap.

Nonhabitual nappers like myself, on the other hand, can fall asleep and stay asleep well enough, but have shown noticeably slower motor functionality upon waking from a nap.

One reason for this has to do with how your homeostatic sleep pressure (HSP), which regulates sleep intensity, aligns with your circadian rhythm, which regulates the timing of sleep.

The longer you're awake, the more your homeostatic sleep pressure rises, making you sleepier until you fall asleep, which allows the pressure to fall.

"If you are a regular napper, you can get these two forces [HSP and your circadian rhythm] into a good rhythm so they are nicely balanced," says Rebecca Spencer, professor of psychological and brain sciences at the University of Massachusetts Amherst. "The problem is few people nap so regularly."

When nonhabitual nappers nap, they often find it difficult to fall asleep at night because they've released HSP during their nap, so the drive for sleep during the time their body typically sleeps diminishes.

This can lead to sleep disorders, which can then lead to more irregular napping, and like that, an unhealthy sleep cycle has been created.

If napping becomes a pattern for people who aren't able to habitually nap, it can throw off their natural circadian rhythm, which influences functions such as digestion and body temperature.

This is also why we tend to feel disoriented and even sick when we're experiencing jet lag — the internal schedule that our body depends on to function well has been altered.

"We know that besides sleep quality and quantity, getting consolidated sleep is also equally important as it ensures that normal sleep architecture is achieved, especially an adequate amount of REM sleep, which is typically seen most in the second half of an eight-hour sleep segment," Shah says.

There are other negative effects of napping that can affect anyone though, no matter how receptive they are to the benefits. For example, excessive napping could be masking a serious illness.

"In [one] study, we looked at only young adults (average age: 29 years) and we saw that increased napping was associated with increased brain inflammation regardless of age, even when overnight sleep duration is taken into account," Spencer says.

Spencer notes that the likely explanation for the brain inflammation is that it correlates with the level of C-reactive protein in the body, which causes one to sleep. This makes sense since we tend to sleep more when we're sick.

The relationship between excessive sleepiness and inflammation also would explain the number of health problems that have been linked to frequent habitual napping including but not limited to increased risk for hypertension, diabetes, depression and cognitive decline.

So if you find yourself feeling overly sleepy lately and taking a lot of naps, it's probably a good idea to see your doctor to rule out the more serious health risks. They may then recommend a sleep expert like Shah.

"The first thing I do with my patients is to get a sleep history that includes time in bed, sleep onset latency (how long it takes someone to fall asleep once they are in bed), final waketime in the morning, wake after sleep onset (time spent awake once you fall asleep)," Shah says. "I then look for signs and symptoms of sleep apnea such as snoring, daytime sleepiness, witnessed pauses in breathing usually reported by bed partner, etc."

If you feel generally well-rested and are just a great nap-taker, however, by all means continue napping. Just don't rub it in with your friends who might not be so lucky.