Baby sleep patterns: A guide for the science-minded parent

© 2008 Gwen Dewar, Ph.D., all rights reserved

This article covers the science of baby sleep patterns. Here you’ll find information on

• How and when babies adapt to natural cycles of day and night

• Baby sleep states and sleep cycles

• Factors that affect arousal (how easily your baby awakens from sleep)

• The myth of “sleeping through the night”

• The ways that babies fall asleep—-an anthropological review

• Stimulation, over-stimulation, and sleep problems

• How your baby’s temperament may affect her sleep patterns, and

• Developmental milestones and other life changes that can disrupt baby sleep patterns.

I focus on babies aged 1 month – 2 years. If you are interested in newborns, see my article on baby sleep patterns during the first 4 weeks of life.

If you’re looking for information about how many hours babies should sleep, see my article on baby sleep requirements.

And for detailed information about sleep disturbances, see my article on infant sleep problems.

Why baby sleep patterns are so different from your own

Young babies are notorious for sleeping—-and waking—-at awkward times. In part, this is because they haven’t yet developed mature circadian rhythms.

At birth, baby sleep patterns aren’t cued to the natural cycle of day and night. It can take several months before babies become fully adapted to the 24-hour day. Until that happens, babies are unlikely to “settle” for lengthy periods during the night.

(For tips on how you can help your baby make this transition, see "How you can help your baby adapt to the 24-hour day" in my article on newborn sleep.)

If you want information about baby sleep training, click here.

But there is another, more fundamental reason for the awkward timing of baby sleep. Babies sleep is just plain different. Compared to adults, babies spend much less time in deep sleep. And babies’ sleep cycles are much shorter.

As a result, babies are much more easily awakened.

Sleep states and sleep cycles: The basis of baby sleep patterns

To better understand how baby sleep patterns differ from your own, it helps to review what happens when an adult falls asleep.

When adults fall asleep, we pass through a series of sleep stages, including bouts of deep (“slow wave”) sleep, and, finally, REM (rapid eye movement) sleep. REM sleep is well-known for its association with dreaming. It is also notable for its pattern of brain activity, which resembles that of the waking brain. Some systems of the body are pretty active, too. Although the skeletal muscles become temporarily paralyzed, the eyes dart around beneath the eyelids, and breathing and heart rates become irregular.

People are least like to awaken while in deep sleep. They are more likely to awaken from REM sleep.

For the average adult, a single sleep cycle—-beginning with stage 1 sleep and ending with REM sleep-—takes about 90-100 minutes. At the end of a cycle, the sleeper will either awaken or pass back into an earlier sleep stage and repeat the cycle.

As we’ll see, baby sleep patterns are very different.

Baby sleep states: How they differ from adult sleep

Compared to adult sleep patterns, baby sleep patterns are a simple affair. Babies have two sleep states—active and quiet. And their sleep cycles are short-—only 50-60 minutes for the first nine months (Jenni et al 2004).

When a newborn baby first falls asleep, she enters into sleep state called “active sleep.” This sleep state is characterized by fluttering eyelids; relatively rapid, irregular breathing; occasional body movements; and vocalizations (grunts or brief cries). Active sleep is the baby-equivalent of REM sleep, and, like adults in REM, babies engaged in active sleep awaken more easily (Anders 1979; McNamara et al 2002).

About half way through the sleep cycle, the newborn passes from active sleep to quiet sleep. As its name suggests, quiet sleep is characterized by slower, more rhythmic breathing, little movement, and no eyelid fluttering.

While in quiet sleep, babies are less likely to be awakened by noise and other disturbances—including a sudden reduction in the oxygen supply (Parslow et al 2003).

Quiet sleep represents the end of the baby’s sleep cycle. When it’s over, babies either begin the cycle again (re-entering active sleep) or they wake up.

As babies mature, quiet sleep begins to differentiate into distinct NREM stages. In addition, their sleep cycles lengthen, and they spend proportionately less time in active, or REM, sleep.

But change is gradual, and it takes several years for children’s sleep patterns to resemble those of adults. Although NREM sleep stages emerge by 6 months, sleep cycles don’t reach the 90-100 mark until children are school-aged (Jenni and Carskadon 2005). And, throughout early childhood, kids spend a lot time in REM sleep. Whereas adults spend only 20% of sleep time in REM sleep, REM represents 50% of sleep time in newborns and 30% of sleep time in three year olds (Anders 1979).

Sleep states and baby sleep patterns: The implications

Babies begin their sleep cycles with about 25 minutes of active sleep, and they complete a sleep cycle in only 50 minutes. This means:

• Compared to adults and older children, babies are light sleepers. They spend proportionally more time in active sleep.

• Babies are especially easily awakened for 25 minutes (or more) after first falling asleep. That’s why it can be so hard to move a sleeping baby without waking him.

• Babies may experience arousals every 50 minutes or so. If you’re lucky, these arousals are only partial and your baby goes back to sleep without a fuss. If you’re unlucky, your baby becomes fully aroused—and let’s you know about it.

If you’re a sleepless parent, active sleep might sound like a bum deal. But it’s likely that active sleep is important for a baby’s brain development (Siegel 2005).

And it’s also possible that active sleep is “safer” sleep, at least for babies who aren't predisposed to SIDS. Experimental research suggests that healthy infants are more likely to wake up if they experience breathing problems during quiet sleep than during active sleep (Parslow et al 2003).

For some babies--those predisposed to SIDS--active sleep is associated with higher risk. A study of babies who later died of SIDS found that these infants were less likely than healthy babies to arouse from active sleep (Kato et al 2003). Active sleep is associated with more frequent episodes of heart rate irregularities and sleep apnea, conditions that are potentially life-threatening if a baby doesn't wake up.

But the main point is this. Babies who are aroused more easily are at a lower risk for SIDS, or sudden infant death syndrome (Horne et al 2002b).

Nonetheless, some babies are aroused more easily than others. Here are some environmental factors that affect baby sleep patterns of arousal.

Factors that affect arousal—and risk for SIDS

Babies are HARDER to arouse (and therefore at greater risk of SIDS) if

• They live with a smoker (Horne et al 2004a)

• They sleep on their stomachs (Horne et al 2001; Kato et al 2006)

• They are sleep-deprived (Franco et al 2004)

• They are recovering from an infection (Horne et al 2002)

• Their faces are covered by bedclothes (Franco et al 2002; Blair et al 2008)

• Room temperature is high (e.g., temperatures of 28C/82F are associated with fewer arousals than are temperatures of 24C/75F—-Franco et al 2001)

Studies show the following conditions PROMOTE AROUSALS and/or LOWER the risk for SIDS:

• Babies are aroused more easily in the supine position (i.e., on their backs; Horne et al 2001).

• Babies who sleep near their mothers experience more arousals (Mosko et al 1997; Mao et al 2004). But certain conditions can make bed sharing risky. See this article about bed sharing safety for the details.

• During active sleep, breastfed babies are aroused more easily than are formula-fed babies (Horne et al 2004b; Franco et al 2000).

• Babies who frequently use pacifiers are aroused more easily (Franco et al 2000).

• Swaddled babies experience fewer startles while they sleep. In addition, they experience shorter arousals during active sleep, which reduces the chance that they’ll wake all the way up. They also have fewer arousals during quiet sleep (Gerard et al 2002). You might think this would put swaddled babies at greater risk of SIDS, and this is indeed the case when swaddled babies are placed on their stomachs. But when placed on their backs, swaddled babies are actually better off. Studies show that swaddled babies sleeping on their back are at a lower risk for SIDS than are unswaddled babies sleeping on their stomachs (Ponsonby et al 1993; Wilson et al 1994). If you swaddle your baby, follow safety guidelines. Keep babies head uncovered (to reduce the risk of overheating) and don’t swaddle too tightly. The baby should be able to move his hips and knees freely, and his chest shouldn’t be constricted.

Sleeping through the night?

If you consider that baby sleep patterns are characterized by short, 50-60 minute sleep cycles, you may wonder how it’s possible for babies to “sleep through the night.”

The answer is that “sleeping through the night” is a myth.

A variety of anthropological, historical, and clinical evidence suggests that adults are not designed to sleep through the night. Let alone babies.

With their short sleep cycles, babies experience more opportunities for arousal. And they have smaller stomachs, which means they need to eat more frequently than adults do.

So when some parents boast that their baby is sleeping through the night, what they’re really saying is that they are not aware of their baby’s night-time arousals. Their baby, in other words, doesn’t make enough noise to awaken them.

How long does this blessed state last? Depending on the age of the baby, maybe no more than 5 hours (e.g., Pinilla and Birch 1993).

Where and how babies fall asleep: cross-cultural baby sleep patterns

Many Western baby “sleep trainers” warn parents against letting their babies fall asleep at the breast or in their parents’ arms. They also warn against rocking or singing babies to sleep. Babies, the sleep trainers argue, must learn to fall asleep alone. They should be trained to “self-soothe,” even if this means babies cry themselves to sleep.

There are limits to these recommendations. For instance, sleep scientists warn that the “cry it out” approach must be carefully monitored and should not be attempted on infants less than 6 months of age (France and Blampied 1999; Owens et al 1999). But in general, the sleep trainers believe that parents should encourage babies to fall asleep by themselves.

From an anthropological standpoint, this is a peculiar notion. Throughout human evolution, babies have fallen asleep while they nursed. They have fallen asleep while they were held or carried—often in the midst of the daily bustle. And they have fallen asleep lying in bed next to their mothers.

Consider this sample of traditional baby sleep patterns:

• From Africa to the Arctic to the Americas, modern-day hunter-gatherers keep their babies in close, physical contact throughout the day, and mothers typically sleep with them at night. Babies frequently fall asleep while breastfeeding and experiencing skin-to-skin contact (Konner 2005).

• In Bali, babies sleep with their mothers until age three. During the day, they are carried in slings as their mothers go about their daily work. Babies may fall asleep in the sling, and, if mothers must put their babies down, they are encouraged to give them to someone else to hold (Diener 2000).

• Among the Beng, village farmers living in the Cote D’Ivoire, babies spend their days on someone’s back—either the mother or a designated baby carrier. Carrying a baby in this way is considered a good way to get babies to fall asleep (Gottlieb 2000).

• In Japan, family members have traditionally slept in the same room, and many babies sleep in their parents’ beds (Fukumizu et al 2005). Where babies sleep in their own beds, parents often lie with babies until they fall asleep (Moore et al 1957).

• In Italy, babies are not usually assigned a special bedtime, and often fall asleep before their parents put them to bed. For the first year of life, a baby often sleeps in his parents’ bedroom (New et al 1996).

• Mayan babies share their mothers’ beds—and may breastfeed during the night—until they are 2-3 years old (Morelli et al 1992).

• Among the Ifaluk of the South Pacific, babies sleep alongside their parents each night. This continues until they are about three years old. During the day, babies may be rocked to sleep (Le 2000).

• In Turkey, babies are often permitted to fall asleep while they nurse. Babies are also swaddled and rocked to sleep in their cradles (Delaney 2000).

Even in the United States—where sleep training is widely touted—it’s not clear how many parents are following the advice of baby trainers. In a recent study that tracked children from 6 months to 4 years of age, most parents soothed their babies to sleep (Gaylor et al 2005).

So why do Western sleep experts promote solitary sleeping for infants? One stated reason is that it encourages emotional independence in children.

However, as anthropologist James McKenna has pointed out, there is no research to support the idea that falling asleep alone makes children more independent (McKenna and McDade 2005). Indeed, the little research that does exist on behavioral outcomes suggests that children who co-sleep with their parents develop higher self-esteem (Okami et al 2002).

A more valid argument for the solitary sleeping regimen is that it promotes quieter babies. Studies confirm that babies who fall asleep by themselves are more likely to go back to sleep by themselves when they awaken in the middle of the night (e.g., Anders et al 1992). For many parents, this can be an important advantage.

But for parents who are concerned about the possible adverse effects of solitary sleep training—-or who just prefer to soothe their babies because it feels intuitively right—-the promise of fewer night wakings is not enough to change their minds.

And—-interestingly—-parents who don’t expect their children to fall asleep by themselves are less likely to report that their children have sleep problems (Morelli et al 1992).

Indeed, parents living in co-sleeping cultures are often disturbed to hear that some Western babies are left to sleep alone. To them, the practice seems tantamount to child neglect (Morelli et al 1992).

Is the custom of solitary sleeping is bad for babies? Some researchers and pediatricians speculate that a rigidly-imposed solitary sleep regimen may stress children, damage parent-child relationships, or cause sleep disturbances (Sears et al 2005; McKenna and McDade 2005). Given that babies vary in their psychological and emotional needs, it seems plausible that at least some babies will suffer problems if forced to follow a solitary sleep regimen. But more research is needed to test these hypotheses.

Meanwhile, the cross-cultural evidence suggests that babies didn’t evolve to fall asleep by themselves. And studies of Western babies show that most babies don’t fall asleep without parental soothing (Gaylor et al 2005; Goodlin-Jones et al 2001).

Stimulation and baby sleep patterns

The total amount of time that babies spend sleeping may vary from culture to culture. For instance, a study conducted in the 1990s reported that 3-month old Dutch infants slept 2 hours more than did American infants at the same age (Super et al 1996).

In part, such differences may reflect different attitudes about stimulation. Whereas American parents feel it is their duty to fill their babies’ days with developmentally stimulating experiences, Dutch parents tend to emphasize rest and regularity (Super et al 1996). Babies that receive lots of sensory stimulation during the day may have greater difficulty falling and staying asleep.

Individual differences in baby sleep patterns

Not surprisingly, baby sleep patterns may differ from individual to individual. A number of studies indicate that a baby’s temperament—how she responds to stimulation and regulates her emotions—is related to sleep patterns.

For instance, studies have shown that babies with “easy” temperaments—for instance, those who are more adaptable, more approachable, less distractible, or who generally are in good moods—go to sleep earlier and sleep more overall than do babies with “difficult” temperaments (Weissbluth and Liu 1983; Van Tassel 1985; Scher et al 1992; Sadeh et al 1994; Scher et al 1998).

Another study also found that babies rated as more active during the day had more frequent awakenings at night (Scher et al 1998).

Baby sleep patterns and development: How major milestones can change sleep

Baby sleep patterns are affected by several important developmental milestones:

• Circadian rhythms. As a newborn, your baby will not be very tuned into the natural daily cycle. Newborns sleep at intervals around the clock. But gradually they will adapt to the rhythms of the 24 hour day, and do most of their sleeping at night. Some babies may reach this point by 3 months. At about the same time, these babies begin “sleeping through the night,” i.e., sleeping for as long as 5 hours at a stretch. Other babies may take much longer—over 5 months—before they reach this milestone (Jenni et al 2006). For tips on how to help your new baby develop her own circadian rhythms, see my article on newborn sleep patterns. • Separation anxiety. Around 6 months of age, your baby’s brain starts to mature in ways that greatly enrich her emotional life (Eliot 1999). She begins to be aware of her own feelings, and she begins to form a strong emotional attachment to her primary caregiver (Eliot 1999). This may explain why many babies that were previously settled at night become more needy. Once your baby starts forming strong attachments, she is more likely to feel separation anxiety. She is more likely to anticipate your departures and protest. And she is more likely to become upset if she awakens and you’re not there.

• Teething. Surprisingly, scientific studies reveal that teething rarely disrupts the sleep of most infants (Wake et al 2000). However, some infants will suffer sleep problems for a few days before and after a new tooth erupts.

• Mastering new motor skills. When babies are learning to crawl or walk or accomplish other milestone feats, the excitement may interfere with sleep. During such periods, babies may be more resistant to bedtime and/or experience more night wakings.

Other factors affecting baby sleep patterns

Baby sleep patterns may be disturbed by a number of organic problems, including

• Ear infections

• Yeast and urinary tract infections

• Parasitic infections (like pinworms)

• Fever

• Respiratory infections

• Acid reflux (Kahn et al 1991)

• Cow's milk allergy (Kahn et al 1989)

• Snoring and sleep apnea

These problems require medical treatment. If you suspect your baby suffers from one of these problems, you should check with your pediatrician. (For more details, see my article on medical problems and their effects on baby sleep patterns.)

But sleep researchers (e.g., France and Blampied 1999) say that most disturbances in baby sleep patterns are caused by environmental or behavioral factors, such as

• Overly-stimulating bedtime rituals or soothing behaviors

• Unrealistic parental expectations about how much sleep the baby needs

• Napping in the late afternoon or early evening

• Too much activity in the hours immediately preceding bedtime

• Too much exposure to artificial lighting during the night (and/or insufficient light exposure during the day)

• Disruptions in family life, such as the birth of a sibling

For more information about these factors--and how to improve baby sleep patterns--see this article on infant sleep problems.

References: What scientific studies reveal about baby sleep patterns

For a complete list of the studies and publications cited in this article, click here.

Content last modified 9/11