Sleep requirements: A guide for the science-minded
© 2008-2010 Gwen Dewar, all rights reserved
This article reviews the science of sleep requirements. It covers
• Why you should be skeptical about “official” sleep recommendations
• The average sleep times for infants, children, teens, and adults
• Information on daytime sleep (naps)
• Guidelines for determining your family’s personal sleep needs
• Problems associated with too little and too much sleep
If you’re looking for detailed information about babies, see my article on
baby sleep needs.
What are your family's sleep requirements?
Even the experts don't know for sure
The most surprising thing about sleep requirements is how little we know about them (Hunt 2003).
The official-looking charts we see published everywhere--the ones telling us that adults need 7 hours of sleep, or that grade schoolers need 10 hours of sleep--are based on how much time the average individual spends in bed.
The charts don’t tell us how much of this time is actually spent sleeping.
Nor do they tell us about how sleep varies cross-culturally. Typically, recommendations about sleep requirements are based on surveys of Western populations(e.g., Iglowstein et al 2003; Armstrong et al 1994; Roffwarg et al 1966).
Most importantly, the charts can't tell us what your individualized needs are.
Knowing how much time people spend in bed is somewhat helpful, but it doesn’t tell us if these people are getting the right amount of sleep.
As the National Center on Sleep Disorders Research has noted, we need new, large-scale, controlled studies that measure both sleep and biological outcomes (Hunt 2003). Unfortunately, such studies are uncommon.
Notable exceptions are recent studies focusing on behavior problems and obesity.
For example, a study of 297 Finnish families with children aged 5-6 years, researchers found that kids who slept less than 9 hours each day had 3-5 times the odds of developing attention problems, behavior problems, and other psychiatric symptoms (Paavonen et al 2009).
Another recent study tracked the development of obesity in young children.
In that study, researchers recorded the body weights and sleep habits of kids under five years of age. Then, five years later, they measured the kids again.
The study revealed a link between sleep loss and obesity. Kids who'd gotten less than 10 hours of nighttime sleep at the beginning of the study were twice as likely to become overweight or obese later on (Bell and Zimmerman 2010).
Moreover, researchers found that the timing of sleep mattered. When it came to reducing the risk of obesity, daytime naps didn't help. For young children, the crucial factor was getting more than 10 hours of sleep at night.
Studies like this are very useful. They give us some concrete guidelines about sleep for young children.
But what about other health markers? What about older kids? And how do we know what's normal?
We can try to answer these questions by consulting the range of sleep times that are typical for many infants, children, and adults (see the tables below).
But keep the following points in mind:
• There is no optimal number of sleep hours that applies to all adults or all kids (Dement and Vaughan 1999; Jenni et al 2007). Sleep requirements are probably influenced by growth rates, stress, disease, pregnancy, and other aspects of your physical condition. They may also be influenced by your genes (Gottlieb et al 2007).
• The most recent scientific study of sleep duration among children reveals a tremendous amount of variation between individuals--especially during early childhood. For example, newborns may sleep anywhere from 9 to 19 hours a day (Iglowstein et al 2003). Kids at both ends of the spectrum may be healthy and normal.
• Sleep patterns vary internationally. People living in Western-style, industrialized societies tend to have less “down-time” than people living non-Western, traditional societies (Worthman and Melby 2002). And children get different amounts of sleep depending on what country they live in. For instance, children in China and Italy sleep less than do children in the Netherlands and the United States (Ottaviano et al 1996; Lui et a 2003; Super et al 1996). Do Chinese and Italian kids get enough sleep? At present, we lack the scientific studies to answer this question (Jenni and O’Connor 2005). Meanwhile, we shouldn’t assume that the “average sleeper” in any given study is getting the optimal amount of sleep. Some populations may be chronically under slept; others may be especially well-rested.
• In the West, average sleep times—-and perceptions of sleep requirements-—have changed from generation to generation (Iglowstein et al 2003; Jenni and O’Connor 2005). For instance, American adults today appear to sleep less than American adults did in the 1960s. And, for no clear scientific reason that I can discern, the medical establishment has adjusted its recommendations accordingly. In the 1960s, experts advised adults to get 8-9 hours of sleep (Roffwarg et el 1966). Today, many medical organizations recommend 7-8 hours. Have American sleep requirements changed since the 60s? It seems unlikely. Until medical recommendations are backed by solid scientific evidence, we ought to be skeptical.
• Too little sleep can harm adults and kids. Scientific studies link childhood sleep loss with fatigue and bad moods (Oginska and Pokorski 2006), attention problems (Fallone et al 2001), academic problems (Fallone et al 2005), and obesity (Lumeng et al 2007; Bell and Zimmerman 2010).
• Sleeping less than average isn’t necessarily bad. Some kids sleep less than others, and they don't always suffer for it. For example, researchers tracking the sleep habits of Swiss children found that individual differences in sleep time were not correlated with differences in growth. (Jenni et al 2007).
• Although some parents underestimate how much sleep their children need, others overestimate. Before imposing any particular sleep schedule on your child, it’s important to determine what your child’s own, individual sleep requirements are. Forcing children to go to bed when they aren’t sleepy can cause
bedtime battles and other behavior problems.
Child sleep requirements: The latest estimates of what’s “normal”
The Zurich longitudinal study (Iglowstein et al 2003)
The table below reports the results of a large, prospective study of 493 Swiss children born in the 1970s.
Study subjects were tracked from 1 month to 16 years of age. Sleep times were based on parent reports. Because parents rarely know precisely when their children fall asleep or wake up—or how much time kids spend awake in the middle of the night—these parental reports probably overestimate the amount of time kids actually slept (Jenni et al 2007).
The table shows average total sleep durations—how much total sleep kids got every 24 hours—for each age group (Iglowstein et al 2003).
It also provides information about the degree of variation.
For example, the table indicates that 96% of two-year olds slept between 10.8 and 15.6 hours. Only 2% got less than 10.8 hours, and only 2% got more than 15.6 hours.
Note: For more detailed information about baby sleep times, see this article on
baby sleep requirements.
What about naps?
The times listed in the table above reflect total sleep duration-—nighttime sleep + daytime sleep. Among these Swiss kids, napping was the practice of young children only. By age 7, less than one percent of the sample population still napped (Iglowstein et al 2003). Before this age, naps were a significant source of sleep time:
• 6 mos: 3.4 hours average; range 0.4 – 6.4 hours
• 9 mos: 2.8 hours average; range 0.2 – 5.3 hours
• 1 year: 2.4 hours average; range 0.2 – 4.6 hours
• 1.5 years: 2.0 hours average; range 0.5 – 3.6 hours
• 2 years: 1.8 hours average; range 0.7 – 2.9 hours
• 3 years: 1.7 hours average; range 0.4 – 2.6 hours
• 4 years: 1.5 hours average; range 0.7 – 2.4 hours
All infants 12 months and under took naps. By age 3, about 50% of kids took naps. Thirty-five percent of 4-year olds took naps (Iglowstein et al 2003).
The Swiss data are probably consistent with practices in other Northern European and Anglo cultures—-cultures where daytime sleep is discouraged for older children and adults (e.g., Iceland: Thorleifsdottir et al 2002).
But the truth is that human beings are very flexible about when and how they meet their sleep requirements.
In many parts of the world, napping is a normal part of life for children and adults (Worthman and Melby 2002). In fact, the historical and anthropological evidence suggests that humans were designed to get their sleep in at least two separate shifts (Worthman and Melby 2003; Ekirch 2005).
So the Swiss study is not representative of kids living in “pro-napping” or “siesta” cultures around the Mediterranean and in Asia, Africa, and the Americas.
For instance, in Saudi Arabia, napping is common among older kids. According to a study of school-age children in Riyadh, 45% of 13-year olds take regular naps (BaHamman et al 2006).
And even in countries where napping is discouraged by the mainstream, specific ethnic groups may encourage napping. In the Southern United States, African-American kids are much more likely to nap—and to nap more frequently—than are European American kids (Crosby et al 2005). In one study, 40% of African-Americans were still taking naps at 8 years of age (Crosby et al 2005).
The authors of the Swiss study report that their results agree with the results of other, older studies of Western children (Iglowstein et al 2003). The average sleep times for Swiss kids are also consistent the National Sleep Foundation’s recommendations about sleep requirements. According to this organization, children should get the following amounts of sleep every 24 hours:
• < 1 year: 14-15 hours
• 1-1.5 years: 14-15 hours
• 1.5-3 years: 12-14 hours
• 3-5 years: 11-13 hours
• 5-12 years: 10-11 hours
• Teens: 8.5-9.5 hours
But contemporary American kids are not meeting these guidelines. According to a telephone survey conducted by the National Sleep Foundation in 2004, these are the average sleep times (i.e., average sleep duration over a 24-hour period) for American kids:
• 6-8 mos: 12.8 hours average
• 9-11 mos: 12.1 “”
• 12-17 mos: 12.5 “”
• 18-23 mos: 11.6 “”
• 2 years: 11.4 “”
• 3 years: 10.8 “”
• 4 years: 10.5 “”
• 5 years: 10.1 “”
• 6-10 years: 9.3-9.8 “”
Are American kids sleeping less than Swiss kids? It’s not clear because the American survey focused on children born later than the Swiss study did.
Possibly, the National Sleep Foundation survey results reflect an international trend towards shorter sleep times for kids.
This seems plausible because total sleep duration among Swiss kids consistently decreased between 1974-1986 (Iglowstein et al 2003). In addition, recent studies in Saudi Arabia (BaHammam et al 2006), Hong Kong (Ng 2005), Australia (Smedje 2007), Israel, and Finland (Tynjala et al 1993) suggest that kids in these countries sleep less than did children in 1974-1986 Swiss cohort.
What about you? Adult sleep requirements
As noted above, beliefs about adult sleep requirements have changed over time.
In the 1960s, the average Western adult was sleeping about 8 hours a day and medical professionals believed that 8-9 hours was ideal (Roffwarg et el 1966).
Today, the Western medical establishment considers 7-8 hours of sleep to be normal. But it’s not clear that 7-8 hours is enough to meet the average adult’s sleep requirements.
People may be sleeping less because they are stimulating themselves with artificial lights after sundown. And in some people, this may lead to a chronic sleep debt.
In an experiment conducted by the National Institute of Mental Health, eight men who normally slept about 7 hours a night were confined to darkness for 14 hours a day (mimicking natural lighting conditions in high latitudes during the winter).
At the beginning of the study, these men began sleeping about 11 hours—as if they were catching up on their sleep. Afterwards, they settled into a new pattern of almost 9 hours of sleep per 24 hours (Barbato et al 1994).
Although it’s hazardous to speculate on the basis of one small experiment, the results suggest that 7-8 hours of sleep may short-change at least some adults.
The anthropological evidence hints of this as well. Before the invention of electric lights, humans-—no matter where they lived—-spent at least half their lives in darkness. Contemporary people who live without electrical lighting don’t spend all that “dark time” sleeping, but darkness restricts them from working (Worthman and Melby 2002). When you consider that people living in these societies are also likely to take naps during the day, it’s pretty clear that people living without electricity get more than 7-8 hours of “down-time.”
Fine-tuning: Your family’s individualized sleep requirements
Sleep charts may give us a rough idea of what is considered normal. But the best guide to your own sleep requirements is how you feel and perform (Dement and Vaughan 1999). There are several ways to take stock of your individualized sleep requirements-—and the individualized sleep requirements of your kids.
According to Stanford researcher and world-renowned sleep expert William C. Dement, the best way to determine your own sleep requirements is to keep a sleep diary. This involves noting the time you go to bed, the approximate time it takes for you to fall asleep, and the time you awaken in the morning. It also involves keeping track of how sleepy you feel during the day (Dement and Vaughan 1999).
You can adopt this approach for your kids, too. In general, you are probably not getting enough sleep if
• You are sleepy at the wrong time of day (e.g., after waking in the morning)
• You have trouble paying attention during the day
• You tend to fall asleep very quickly (within a few minutes) when given the chance
• You are “wired” at the wrong time of day (e.g., just before bedtime)
These principles apply to kids, too. Studies reveal other symptoms of sleep deprivation in kids (Dahl 1996), including:
• The child is easily frustrated and quickly irritated
• The child has trouble keeping his impulses in check
For more help determining your family's individualized sleep requirements,
You’ll find more details about Dement’s approach, as well as a guide to signs of sleep deprivation in babies and young children.
When you fail to meet your sleep requirements: The health consequences
You might assume that medical pronouncements about sleep requirements are based on health outcomes. But there is no solid, scientific evidence for an optimal number of hours.
What we do know is this. In controlled studies of sleep restriction, people who get very little sleep (typically, 4 hours or less) suffer the following problems:
• Impaired attentiveness (Fallone et al 2001)
• Impaired ability to retain new memories (Yoo et al 2007a)
• Impaired immune system (Rogers et al 2001)
• Greater emotionality (e.g., becoming more upset by disturbing images—Yoo et al 2007b)
• Increased afternoon and evening cortisol (stress hormone) levels (Copinschi 2005)
• Increased feelings of hunger (which may lead to overeating—Copinschi 2005)
Long-term health problems are another matter. Recent studies have shown a correlation between sleep duration and mortality. A study of American adults (ranging from 30-102 years old) showed that people who habitually slept about 7 hours a night had the best survival rates.
People who reported sleeping less than 6 hours a night—or more than 8 hours a night—were more likely to die (Kripke et al 2002). Interestingly, a separate study of Japanese adults (between 40-79 years of age) had similar results: Sleeping more or less than 7 hours was associated with higher mortality (Tamakoshi and Ohno 2004).
This research got a lot of media attention when it was published, and many headlines implied that there was a causal link between sleep duration and mortality.
But we can’t yet draw any conclusions about causation. As the study authors noted, their research design can’t tell us why people who get more or less sleep are at higher risk. Habitually “short” and “long” sleepers may suffer underlying health problems that cause both sleep disturbances AND increased mortality.
For instance, people with sleep apnea are less efficient sleepers, and may have to sleep longer hours in order to achieve minimal levels of alertness during the day. But sleep apnea patients are also more likely suffer dangerous health problems, and they are at greater risk of dying while they sleep. Other life-threatening medical conditions may cause people to sleep longer or shorter than average, resulting in a correlation between long sleep duration and mortality.
The bottom line? Sleeping more or less than average may be a symptom of an underlying health problem that causes increased mortality. But it may also reflect your perfectly healthy, individually-determined sleep needs. If you habitually sleep much less or much more than average, you might want to have your doctor check you for such health problems as hear disease, sleep apnea, and depression.
References: What scientific studies suggest about human sleep requirements
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Content of "Sleep requirements" last modified 9/11
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