“At one time we went to bed in the hours after dusk and woke up with the chickens,” the neuroscientist Matthew Walker writes in his book Why We Sleep: Unlocking the Power of Sleep and Dreams. Now, Walker says, we’re still getting up at the same time, but when night falls, we’ve only just finished our working day and will stay awake for a while longer. Modern society, he explains, has made it much more difficult for humans to stick to the sleep pattern we’re genetically programmed for: biphasic sleep, which involves a long period of rest at night, followed by a shorter one during the day. “All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours,” says Walker, who is the director of UC Berkeley’s Sleep and Neuroimaging Lab. And napping is the solution to that phenomenon. Communities in which a daytime snooze is a common habit “have sometimes been described as ‘the places where people forget to die,’” the neurologist says. “From a prescription written long ago in our ancestral genetic code, the practice of natural biphasic sleep, and a healthy diet, appear to be the keys to a long-sustained life.”
Napping has an impact on our health — for better or for worse. The scientific community is not yet completely clear on the full extent of its influence, but the results of medical research on the subject point to one key takeaway: less is more. A study published last month in the journal Obesity has found that people who have longer snoozes have a 23% higher risk of being overweight. Those who take only a short nap, meanwhile, are less likely to suffer from high blood pressure. The investigation’s authors agree that a short doze (up to 30 minutes) can be restorative and help improve alertness. In addition, they say the nap shouldn’t take place in bed, and ought not to be any later than lunchtime or early afternoon.
Every nap is different. In the scientific community, where the topic sparks huge debate and, frequently, conflicting studies are published on the impact of a particular variable, researchers have learned to fine-tune their conclusions by focusing on how long naps last and what particular population group they’re looking at. A brief snooze on the sofa isn’t the same as a multi-hour, late-afternoon sleep in bed. There are also differences between naps taken by a person with a history of obesity and by a healthy individual, or between snoozing by a young adult and someone who is older. “For example, we recommend preventive, 10-minute naps to people with narcolepsy,” says Francesca Cañellas, a member of the task force on insomnia at the Spanish Sleep Society. “And people with sleep apnea also need naps to remain active. Insomniacs would love to have a nap, but they’re incapable.”
At each stage of our lives, we have a different sleep cycle: adolescents need more hours of rest, for instance, while people in old age have a nighttime sleep pattern that’s more fragmented. A study in the journal Sleep Medicine Reviews maintains that, among young adults, a nap of around 20 minutes has immediate, positive effects on their alertness and doesn’t affect their nighttime sleep; in older people, snoozes of around half an hour don’t have consequences for how well they sleep at night, either, but long naps do. That’s why senior citizens are urged to keep them short if they have them, and to “take good care of their circadian rhythms”, Cañellas says. That means respecting your body clock’s natural routine and exposing yourself to sunlight during the day, so that your body knows it has to be active.
The paper published in Obesity, which studied 3,300 participants in Spain, looked into napping’s influence on metabolic health and found an association between the duration of these rest periods and certain metabolic indicators: people who had long naps — more than 30 minutes — had a body-mass index (BMI) that was 2% higher than those who didn’t sleep during the day, and their risk of obesity increased by 23%; meanwhile, those who had short naps (no more than half an hour) were 21% less likely to suffer from high systolic blood pressure. “There was only an association with obesity in long naps,” explains Marta Garaulet, one of the study’s co-authors and a professor in physiology at the University of Murcia.
Better not to nap in bed
Garaulet and her team also found that people who had long naps tended to do so in bed, rather than on the sofa or in an armchair. “If you nap in bed, there is a greater association with hypertension than if you stay on the sofa,” says Garaulet, who is also a visiting professor at Harvard University. “It seems to be related to changes in posture, but we’re not completely sure.” As she outlines in another study, published in Nature Communications, Garaulet has also discovered that there are 123 genes specifically related to napping, which may explain why some people find it easier to have a daytime doze than others.
Garaulet points to a number of physiological explanations that back up the Obesity study’s results on napping’s health impacts. “We’re designed to sleep at night and be awake during the day,” she says. “If you sleep during the day and reach a deep state of sleep, you’re fooling your body into believing that it’s nighttime. When you limit yourself to a short nap, you only reach sleep stages 1 or 2, which are relaxing but don’t count as deep sleep.” What’s more, she adds, a long nap can serve to delay nighttime sleep: “It leads you to have a later chronotype: you have your evening meal later and you go to bed later. This is a circadian factor that contributes to ill health and obesity.” Garaulet also explains that extended naps can bring about “an alteration in your cortisol rhythm, which is associated with stress.”
In line with Garaulet’s research, other studies are also focusing on the impact of nap duration. An investigation into postmenopausal women has linked those who snooze for longer than 30 minutes to a 74% higher chance of developing diabetes than in those who don’t nap. Meanwhile, a study by the American Academy of Neurology has identified an association between naps of more than an hour and a greater stroke threat: the probe found an 88% higher risk than in people who did not have such daytime rest periods. Another scientific review on the dangers of extended naps has warned, like Garaulet’s study, that a risk of long snoozes is that they “alter sleep latency, and the quality and quantity of subsequent nighttime sleep.” Garaulet concludes: “A short nap can be restorative, but doesn’t involve the kind of deep sleep that leaves you groggy.”
“Long naps tend to affect people’s sleep and wakefulness cycles, and that’s a bad habit,” Cañellas agrees. Mercé Mayos, the head of the sleep unit at Barcelona’s Hospital Sant Pau, acknowledges that there are still “many questions to be answered about napping,” noting that there is contradictory information out there, but concludes: “A high percentage of the literature on napping finds that a [short] nap has particular benefits.” And she lists them: you’re more relaxed, are more alert, have lower reaction times, and are in a better mood. “Extended naps, taken in bed and lasting more than an hour, are associated with health problems such as obesity and metabolic syndrome, as Garaulet’s study says,” Mayos explains.
However, research on napping’s influence on our health tends to point towards associations, rather than outright causes and effects. Mayos stresses, indeed, that doubts can be sown over potentially “confounding factors.” “To what extent is there a causal or coincidental relationship?” she asks. “Other lifestyle factors may be involved, or other health problems,” Mayos concedes. In her research, Garaulet has found habits that have a role in the association between napping and obesity, but, for example, she doesn’t know whether the chicken or the egg came first. “We’re not sure whether you have a snooze because you eat more, or vice versa,” she admits.
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