Sleep requirements in children
© 2008-2014 Gwen Dewar, all rights reserved
What are your child's sleep requirements? Even the experts don't know for sure.
Sleep has a big impact on our well-being, so it's understandable that parents want to know if their kids are getting enough. Recent research suggests that something as simple as a well-timed nap makes a difference in how much preschoolers learn (Kurdziel et al 2013). Naps may also enhance learning in babies.
But while it's clear that sleep is important, there is no easy
formula for calculating your child's personal sleep needs. In fact, the
most surprising thing about sleep requirements is how little we know
about them (Hunt 2003).
The official-looking recommendations we see everywhere, like the ones in the box below from National Sleep Foundation, are often based on studies of how much time people spend 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., Blair et al 2012; 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
As the National Center on Sleep Disorders Research has noted, we
need large-scale, controlled studies that measure both sleep and
biological outcomes (Hunt 2003). Unfortunately, such studies are
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.
Is the evidence conclusive? No. Some research has failed to find
links between sleep time and fat accumulation, like one study of
children under the age of 3 (Klingenberg et al 2013), and of course we
can't be sure about causation. Some kids may suffer from medical
conditions that cause both sleep problems and obesity. Perhaps in the
near future investigators will resolve these discrepancies.
Meanwhile, 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 cross-culturally. In some cases, cultural differences are mostly about the scheduling
of sleep. For instance, in predominantly Asian countries, preschool
children get less sleep at night than do kids in predominantly Caucasian
countries, but they make up the shortfall by napping during the day
(Mindell et al 2013). In other cases, cultural differences concern the total amount of sleep
people get over a 24-hour period. Kids in China and Italy appear to get
less sleep than do children in the Netherlands and the United States
(Ottaviano et al 1996; Lui et a 2003; Super et al 1996). Who is better
off? At present, we lack scientific studies that address 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
• 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
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 have serious health consequences.
Scientific studies link childhood sleep loss with fatigue and bad
moods(Oginska and Pokorski 2006; Berger et al 2012), attention problems
(Fallone et al 2001), impaired memory consolidation (Kurdziel et al
2012), 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
bedtime battles and other behavior problems.
What's typical today?
The latest clues from a study of British kids
The table below reports the results of a large, prospective study of
more than 11,500 kids born in South-West England in 1991-1992.
At 8 different points in time – beginning when the children were 6
months old and ending when they were 11 years old -- Peter Blair and
his colleagues (2012) asked parents when their kids “normally went to
bed in the evening and woke in the morning on an average weekday.”
From these data, the researchers calculated how much time kids spent in bed
each night. 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 the average nighttime sleep duration for each age group (Blair et al 2012).
It also provides information about the degree of variation.
For example, the table indicates that 95% of preschoolers (aged
approximately 3.5 years) slept between 10.4 and 12.2 hours. Only 2.5%
got less than 10.3 hours, and only 2.5% got more than 12.3 hours.
Note: For more detailed information about baby sleep times, see this article on
baby sleep requirements.
What about naps?
The researchers also asked about daytime sleep. Among children who
napped, how much time did they log in bed? Here are the results:
• 6 months: 2.4 hours average; range 1.3 – 3.5 hours
• 18 months: 1.5 hours average; range 1 – 2 hours
• 30 months: 1.2 hours average; range 0.7 – 1.7 hours
• 3.5 years: 1.1 hours average; range 0.5 – 1.7 hours
• 5 years, 9 months: 1 hours average; range 0.3 – 1.7 hours
• 6 years, 9 months: 1.2 hours average; range 0.3 – 2.1 hours
Virtually all children 18 months and younger were reported to
take naps. Most children (59%) still took naps at the age of 30 months.
But by the time kids were 3.5 years of age, only 23% slept during the
day, and reports of napping were very uncommon for kids older than 5
The British 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, Switzerland: Iglowstein et al 2003).
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 British 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).
Is the early abandonment of napping detrimental? For some kids it
might be. Recent experimental research indicates that toddlers who skip
naps are more likely to (1) show confusion and negative emotion in
response to challenging tasks (Berger et al 2012) and (2) have trouble
"downloading" new information into long-term memory (Kurdziel et al
Are these "average" kids meeting their sleep requirements?
The British study tells us about what's normal among a certain
population of children. But what's normal keeps changing. As the authors
note, their results differ substantially from the results of older,
earlier studies of Western children.
For instance, a major study of Swiss kids born in the 1970s
reported that babies and toddlers got almost an hour more sleep each
night than did the more contemporary, British children (Iglowstein et
The modern British kids are also falling short of the National
Sleep Foundation's recommendations about sleep requirements (as noted
above). And that's in keeping with an international trend towards
shorter sleep times for kids.
From the United States to Saudi Arabia to Hong Kong, recent
studies indicate that kids are sleeping less than experts typically
recommend (Matricciani et al 2012; BaHammam et al 2006; Ng 2005 Smedje
2007; Tynjala et al 1993).
Is this worrying? As I note above, it's hard to say how much
sleep the average child needs for optimal health. We need more rigorous,
focused research to answer that question. But given evidence to date
that links shorter sleep duration with obesity (mentioned above),
attention problems, emotional problems, and impaired academic
performance (Vriend et al 2013; Li et al 2013), I think we should be
concerned. Some kids may find their personal needs are in-sync with the
modern trend towards shorter sleep times. But others may not.
What about adults?
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
We may need more rigorous studies about the optimal duration of
sleep. But there is no lack of evidence regarding the consequences of
severe sleep restriction. In controlled, experimental studies,
volunteers assigned to live on very little sleep (typically, 4 hours or
less) suffered from 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)
Other, correlational research hints at long-term problems for people who deviate from the the modern norm.
For example, 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 either
(1) sleeping less than 6 hours a night, or
(2) sleeping 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 to 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
heart disease, sleep apnea, and depression.
References: What scientific studies suggest about human sleep requirements
Armstrong KL, Quinn RA, and Dadds MR. 1994. The sleep patterns of normal children. Med Journal of Australia 161: 202-206.
Ashworth A, Hill CM, Karmiloff-Smith A, and Dimitriou D. 2013.
Sleep enhances memory consolidation in children. J Sleep Res. 2013 Dec
16. doi: 10.1111/jsr.12119. [Epub ahead of print]
BaHammam A, Bin Saheed A, Al-Faris E, and Shaihk S. 2006. Sleep
duration and its correlates in a sample of Saudi school children.
Singapore Medical Journal 47: 875-81.
Barbato G, Barker C, Bender C, et al.1994. Extended sleep in
humans in 14 hour nights (LD 10:14): relationship between REM density
and spontaneous awakening. Electroencephalogr Clin Neurophysiol.
Bell JF and Zimmerman FJ. 2010. Shortened Nighttime Sleep
Duration in Early Life and Subsequent Childhood Obesity. Arch Pediatr
Adolesc Med. 164(9):840-845.
Berger RH, Miller AL, Seifer R, Cares SR, LeBourgeois MK. 2012.
Acute sleep restriction effects on emotion responses in 30- to
36-month-old children. J Sleep Res. 21(3):235-46.
Blair PS, Humphreys JS, Gringras P, Taheri S, Scott N, Emond A,
Henderson J, and Fleming PJ. 2012. Childhood sleep duration and
associated demographic characteristics in an English cohort. Sleep
Copinschi G. 2005. Metabolic and endocrine effects of sleep deprivation. Essent Psychopharmacol. 6(6): 341-347.
Crosby B, LeBourgeois MK, and Harsh J. 2005. Racial differences
in reported napping and nocturnal sleep in 2 to 8-year-old children.
Pediatrics 115: 225-232.
Dahl RE 1996. The impact of inadequate sleep on children's daytime cognitive function. Seminars in Pediatric Neurology 3: 44-50
Dement W and Vaughan C. 1999. The promise of sleep. New York: Random House.
Ekirch AR. 2005. At Day's Close: Night in Times Past. New York: WW Norton.
Fallone G, Acebo C, Arnedt JT, Seifer R, and Carskadon MA. 2001.
Effects of acute sleep restriction on behavior, sustained attention, and
response inhibition in children. Percept Mot Skills 93: 213-229.
Fallone G, Acebo C, Seifer R, Carskadon MA. 2005. Experimental
restriction of sleep opportunity in children: Effects on teacher
ratings. Sleep 28(12): 1561-1567.
Gais S, Lucas B and Born J. 2006. Sleep after learning aids memory recall. Learning and Memory 13: 259-262.
Galland BC, Taylor BJ, Elder DE, Herbison P. 2012. Normal sleep
patterns in infants and children: a systematic review of observational
studies. Sleep Med Rev. 16(3):213-22.
Gottlieb DJ, O’Connor GT, and Wilk JB. 2007. Genome-wide
association of sleep and circadian phenotypes. BMC Medical Genetics
8(Supplement 1): S9-S16.
Gruber R, Laviolette R, Deluca P, Monson E, Cornish K, and
Carrier J. 2010. Short sleep duration is associated with poor
performance on IQ measures in healthy school-age children. Sleep Med.
Hunt CE. 2003. National sleep disorders research plan. Bethesda, MD: National Center on Sleep Disorders Research.
Iglowstein I, Jenni OG, Molinari L, Largo RH. 2003. Sleep
duration from infancy to adolescence: Reference values and generational
trends. Pediatrics 111(2): 302-307.
Jenni OG, Molinari L, Caflish JA, and Largo RH. 2007. Sleep
Duration From Ages 1 to 10 Years: Variability and Stability in
Comparison With Growth. Pediatrics 120(4): e769-e776.
Jenni OG and O’Connor BB. 2005. Children’s sleep: An interplay between culture and biology. Pediatrics 115: 204-215.
Jenni OG, Zinggeler F, Iglowstein I, Molinari L, and Largo RH. 2005. Pediatrics 115(1): 233-240.
Klingenberg L, Christensen LB, Hjorth MF, Zangenberg S, Chaput
JP, Sjödin A, Mølgaard C, and Michaelsen KF. 2013. No relation between
sleep duration and adiposity indicators in 9-36 months old children: the
SKOT cohort. Pediatr Obes. 8(1):e14-8.
Kripke DF, Garfinkel L, Wingard DL, Klauber MR, Marler MR 2002.
Mortality associated with sleep duration and insomnia. Arch Gen
Kurdziel L, Duclos K, and Spencer R. 2013. Sleep spindles in
midday naps enhance learning in preschool children. PNAS (epub ahead of
print) doi: 10.1073/pnas.1306418110.
Kurdziel L, Duclos K, and Spencer R. 2013. Sleep spindles in
midday naps enhance learning in preschool children. PNAS (epub ahead of
print) doi: 10.1073/pnas.1306418110.
Largo RH and Hunziker UA. 1984. A developmental approach to the
management of children with sleep disturbances in the first three years
of life. Euro Journal of Pediatrics 142: 170-173.
Lavigne JV, Arend R, Rosenbaum D et al. 1999. Sleep and behavior
problems among preschoolers. Journal of Dev Behav Pediatr. 20: 164-169.
Leproult R, Copinschi G, Buxton O, and Van Cauter E. 2007. Sleep
loss results in an elevation of cortisol levels the next evening. Sleep
Li S, Arguelles L, Jiang F, Chen W, Jin X, Yan C, Tian Y, Hong X,
Qian C, Zhang J, Wang X, Shen X. 2013. Sleep, school performance, and a
school-based intervention among school-aged children: a sleep series
study in China. PLoS One. 8(7):e67928.
Lui X, Lui L, and Wang R. 2003. Bed sharing, sleep habits, and
sleep problems among Chinese school-aged children. Sleep 26: 839-844.
Lumeng JC, Somashekar D, Appulgliese D, Kaciroti N, Corwyn RF,
and Bradley RH. 2007. Shorter sleep duration is associated with
increased risk for being overweight at ages 9 to 12 years. Pediatrics
Matricciani LA, Olds TS, Blunden S, Rigney G, Williams MT. Never
enough sleep: a brief history of sleep recommendations for children.
Maquet P. 2001. The role of sleep in learning and memory. Science 294: 1048-1052.
Mednick S, Nakayama K, and Stickgold R. 2003. Sleep-dependent
learning: a nap is as good as a night. Nature Neuroscience 6: 697-698.
Mindell JA, Sadeh A, Kwon R, Goh DY. 2013. Cross-cultural
differences in the sleep of preschool children. Sleep Med.
Ng DK, Kwok KL, Cheung JM, et al. 2005. Prevalence of sleep
problems in Hong Kong primary school children: a community-based
telephone survey. Chest 128: 1315-1323.
Oginska H and Pokorski J. 2006. Fatigue and mood correlates of
sleep length in three age-social groups: School children, students, and
employees. Chronobiol. Int. 23: 1317-1328.
Ottaviano S, Giannotti F, Cortesi F, Bruni O, Ottaviano C. 1996.
Sleep characteristics in healthy children from birth to 6 years of age
in the urban area of Rome. Sleep 19: 1-3.
Paavonen EJ, Porkka-Heiskanen T, and Lahikainen AR. 2009. Sleep
quality, duration and behavioral symptoms among 5-6-year-old
children.Eur Child Adolesc Psychiatry. 2009 Dec;18(12):747-54.
Rogers NL, Szuba MP, Staab JP, Evans DL, and Dinges DF. 2001.
Neuroimmunologic aspects of sleep and sleep loss. Semin. Clin.
Neuropsychiatry 6(4): 295-307.
Roffwarg HP, Muzic JN, and Dement WC. 1966. Ontogenetic development of the human sleep-dream cycle. Science 152: 604-619.
Shur-Fen Gau S. 2006. Prevalence of sleep problems and their
association with inattention / hyperactivity among children aged 6-15 in
Taiwan. Journal of Sleep Research 5(4): 403-414.
Smedje H. 2007. Australian study of 10- to 15-year olds shows
significant decline in sleep duration between 1985 and 2004. Acta
Paediatrica 96 (7): 954–955.
Super CM, Harkness S, and van Tijen N. 1996. The three R’s of
Dutch childrearing and the socialization of infant arousal. In S.
Harkness & C.M. Super (Eds.), Parents’ cultural belief systems:
Their origins, expressions, and consequences (pp. 447–466). New York:
Super CM and Harkness S. 1982. The infant's niche in rural Kenya
and metropolitan America. In: Cross-cultural Research at Issue, pp.
47-55, L.L. Adler (Ed.). New York: Academic Press.
Tamakoshi A and Ohno Y. 2004. Self-reported sleep duration as a
predictor of all-cause mortality: results from the JACC study, Japan.
Thorleifsdottir , Bjornsson JK, Benediktsdottir B, Gislason T,
and Kristbjarnarson H. 2002. Sleep and sleep habits from childhood to
young adulthood over a 10- year period. Journal Psychosom Res 53:
Toth LA. 2001. Identifying genetic influences on sleep: AN
approach to discovering the mechanisms of sleep regulation. Behavior
genetics 31: 39-46.
Tynjala J, Kannas L, and Valimaa R. 1993. How young Europeans sleep. Health Educ Res 8: 69-80.
Vriend JL, Davidson FD, Corkum PV, Rusak B, Chambers CT,
McLaughlin EN. 2013. Manipulating sleep duration alters emotional
functioning and cognitive performance in children.
J Pediatr Psychol. 2013 Nov;38(10):1058-69.
Worthman CM and Melby M. 2002. Toward a comparative developmental
ecology of human sleep. In: Adolescent Sleep Patterns: Biological,
Social, and Psychological Influences, M.A. Carskadon, ed. New York:
Cambridge University Press, pp. 69-117.
Yoo SS, Gujar N, Hu, Jolesz FA, and Walker MP. 2007a. The human
emotional brain without sleep—a prefrontal amygdale disconnect. Current
Biology 17(20): 877-878.
Yoo SS, Hu PT, Gujar N, Jolesz FA and Walker MP. 2007b. A deficit
in the ability to form new human memories without sleep. Nat Neurosci
Content of "Sleep requirements" last modified 1/14
Photo credits for sleep requirements article:
image of yawning boy ©iStockphoto.com/Bronwyn8
image of eagle owl ©iStockphoto.com/Dirk Freder