What Is Sleep?

Falling asleep, staying peacefully asleep, long and healthy naps—all are rooted in your child’s biology. When you understand a few basic principles of how sleep works, you can use this knowledge to help your child sleep better at any age.

In this chapter, we’ll take you through the basics of sleep science so you can:

1. Understand the underlying mechanisms of sleep.

2. Use this understanding to set your child’s sleep schedule and improve naps and nighttime sleep.

Just like language or motor skills, sleep matures over the first years of your child’s life. The biology of newborn sleep is distinct from older baby sleep, which is different from preschool sleep . . . and so on. As your child grows, the patterns and physiology of her sleep will change—knowing what’s going on in her brain and body will make you better able to support her sleep development.

The Two Processes That Regulate Sleep

You’ve probably watched your newborn baby flicker awake and drift off again almost instantly and randomly throughout the day. This switch from sleep to wake to sleep again is controlled by a basic brain mechanism that acts much like a seesaw, moving your newborn in and out of sleep through the day and night.

As she grows, two main processes begin to regulate her sleep (and continue to do so for her life): the homeostatic sleep drive and the circadian system. The sleep drive is like pressure that builds the longer you’re awake, and then releases while you sleep. The circadian system is made up of internally driven rhythms that are approximately 24 hours in length. You’ll see here how these systems interact and influence when and how long your child sleeps, and how the best sleep will happen when the two are coordinated.

The Homeostatic Sleep Drive

The homeostatic sleep drive is very simple to understand: The longer you’re awake, the greater the drive toward sleeping. For adults, after a day of being awake, the pressure is great enough that you become drowsy and fall asleep (or if you didn’t get enough nighttime sleep, the sleep drive might be more intense and allow you to nap). The sleep drive in babies and children is very strong and builds quickly.

The sleep drive has a powerful influence over your baby’s nap schedule because babies and toddlers build up pressure to sleep over just a short amount of time. If they don’t nap sufficiently, they can become irritable, less capable of learning, and more resistant to falling asleep. Napping relieves this pressure.

This is why your baby is often ready for sleep after 60 to 90 minutes of being awake, and your toddler after 2 to 3 hours. Parents are sometimes astounded that their baby could sleep for 12 hours, be awake for 60 minutes, and fall asleep again. But even with a good night of sleep, the sleep drive quickly builds during the daytime hours.

PRACTICAL TAKEAWAYS

Don’t be surprised by your baby’s need for frequent naps—sleep drive builds very quickly in babies and young children. Be aware of your baby’s optimal awake span between sleeps. Put your young baby down to sleep after each 90-minute span of awake time during the day.
Even a short nap (10 minutes or less) reduces the sleep drive. Keep this in mind when your child sleeps in the car or stroller—it can make it harder for her to fall asleep for a regular nap, since the sleep drive has already been relieved. With a toddler or older child, taking a brief nap in the late afternoon can make it harder to fall asleep at bedtime.
Napping continues to be very important for children until around the end of preschool, in part because their sleep drive is strong. If children don’t nap, they are less able to regulate emotions, learn, and think creatively.

The Circadian System and the Internal Clock

If the sleep drive were acting alone, you would get progressively sleepier and sleepier over the course of the day. That’s not the whole story, though. The other system regulating your child’s sleep (and your own) begins with a powerful internal clock, nestled in the brain. This clock is a biological pacemaker that controls the circadian rhythms (roughly 24-hour cycles), allowing us to anticipate the light-dark pattern of day and night. It’s the same type of clock that causes a morning glory to open with the sunlight or birds to sing in the morning.

The internal clock sends us messages throughout the 24-hour day, influencing our sleep-wake pattern and many other biological processes. As night falls, body temperature begins to go down and levels of the drowsy-making chemical melatonin rise in preparation for sleep. Toward morning, the activating chemical cortisol rises. During the daytime, the circadian system opposes the sleep drive with activating signals (so that you don’t just get sleepier as the day goes on). There are two peaks in alertness in the day, one in the morning and one in the early evening. In between these two peaks is when most adults feel a little drowsy, usually in the mid-afternoon. This is when children who nap once a day take their nap. At bedtime, alerting signals from the circadian system are withdrawn, sleep drive takes over, and the circadian system helps us stay asleep, even as the sleep drive becomes less strong as the night goes on.

When your child wakes up in the morning—experiencing the first rays of sunlight and starting to eat and play—it’s akin to pressing “start” on her internal clock. From this point forward, when she’s drowsy for her first nap and when she’s ready for bed later that evening will be influenced by when this start button was pressed. Your child’s rhythm is also influenced by regularity. Going to bed at the same time every night, waking up at the same time every morning, and napping on roughly the same schedule every day keeps the circadian system in sync and helps her be alert and drowsy at the proper times.

The circadian system: Light picked up by the eye travels along the optic nerve to a tiny cluster of cells in the brain’s hypothalamus, called the suprachiasmatic nucleus (SCN). This process keeps us on a 24-hour rhythm and influences factors such as when melatonin (sometimes called the “hormone of darkness”) rises in the evening to make us drowsy, when we feel alert and productive, when we’re hungry, our mood, body temperature, and more. The SCN is the master clock that controls circadian rhythms, as well as the other clocks in cells throughout the body, so they’re all in sync.

What Time Your Child Is Alert and What Time She’s Drowsy

The circadian system naturally makes your child alert or drowsy at different points during the day. Interestingly, the drive toward sleep is quite high in the morning at your child’s wake time, and it continues to be high for another few hours (for example, from 6:00 to 9:00 a.m. for a child who normally wakes up at 6:00 a.m.). This is called the circadian sleep maintenance zone, and it’s why babies and young children can easily nap soon after waking up.

At the other end of the day, there is a wake maintenance zone, also known as the “forbidden zone” for sleep. This is a period a few hours before your child’s regular bedtime, when it’s harder for her to fall asleep. This is a second wind, a time when your child’s circadian system is making her more alert (in order to counteract the homeostatic sleep drive). If you notice that your child seems sleepy in the afternoon but around dinnertime miraculously gets her energy back, this is the forbidden zone in action, winning out over the sleep drive. If your child has a regular bedtime with a calming routine and dim lighting, her circadian system will withdraw its alerting signals and she’ll become drowsy at that bedtime—now the two forces are working together. Since the circadian system drives alertness a few hours before bed, this may be why babies usually have a longer period of awake time at the end of the day, and why they tend to take shorter naps or be a little fussy in the late afternoon.

The internal clock is accurate, but it doesn’t keep to exactly 24 hours on its own (the average clock length for adults is 24.2 hours). Therefore, we have to reset it each day and keep it at 24 hours with exposure to sunlight in the day and darkness at night; this process is called “entrainment.” The internal clock has to be somewhat flexible because the length of the day changes with the seasons (a clock set to wake you with the sun in April would get you up well past sunrise in May). That is why it’s built so that it can be constantly readjusted (entrained) by external cues. All this is vitally important when it comes to understanding sleep patterns that can be very precise (your baby wakes up almost on cue), yet subject to getting out of sync when external cues (lights on at night, too much activity by other family members, and so forth) are changed.

WHEN THE INTERNAL CLOCK DEVELOPS

A fetus does have a rudimentary circadian system and shows daily rhythms (for example, in heart rate and activity levels) sometime in the second trimester. But this is thanks to the signals of Mom’s hormones (such as cortisol and melatonin), not the baby’s own internal clock.

Rest-activity pattern of a newborn. Dark bars represent sleep. Adapted with permission from Rivkees et al.

When your baby enters the world (and is no longer influenced by Mom’s hormones), he loses his sense of day and night. A newborn’s circadian system on its own is immature. Brand-new babies tend to sleep on and off without much of a day-night pattern; their sleep is fragmented. But every day after he enters the world, your baby’s sense of a daily rhythm continues to develop (research suggests that even a newborn’s circadian system is sensitive to light). At about 6 weeks, infants start to have an increase in evening melatonin (which makes them drowsy), and at this age most infants are more active and awake during the day than they are at night.

By around 2 to 3 months of age, babies have more pronounced rhythms of sleeping and waking and their natural sleep onset is more or less coupled to sunset (this is why we encourage you to put your baby to bed early). They also now have a circadian pattern of core body temperature—this is important because low body temperature at night is a natural component of sleep. All these factors are good news for sleep development, and as time goes on, your baby continues to consolidate sleep at night and be awake for longer periods during the day.

Artificial Light, Electronic Gadgets, and Sleep

Modern life is not very sleep friendly! While our bodies are naturally programmed to sleep as night falls, artificial lights keep us awake and entertained much later.

Light suppresses melatonin, a chemical that normally rises as we become drowsy and ready for sleep. In particular, short-wavelength light (which appears as blue and is emitted by many electronic screens) is the most disruptive to sleep—this kind of light has been shown to suppress melatonin and delay sleep onset. If there are bright household lights or your family watches screens and electronic gadgets up close in the hours close to bedtime, it can make it harder for you to become drowsy. Artificial light can affect sleep in people of all ages.

PRACTICAL TAKEAWAYS

You can’t rush the development of your newborn’s circadian rhythms. Regular sleep takes time (roughly 4 to 6 months) because your baby’s nervous system is naturally maturing. You can help the process along by exposing her to indirect sun in the day and keeping light and activity low at night.
Your child is exquisitely attuned to light cues. Use this to help him set his body’s clock by getting up at roughly the same time every morning, napping at the same time, and going to bed at the same time every night.
Get out and exercise during the day.
Manage artificial light in the evenings so that you don’t confuse your child’s biological clock and suppress melatonin. Dim the indoor lights 60 minutes before bed, keep your child from using electronic devices such as tablets and computers before bed, and keep electronics out of your child’s room. If you have a nightlight, make sure it’s very dim and ideally use a red bulb (red light does not have the strong affect on circadian rhythms that blue light does). Don’t turn on bathroom lights in the night; use a nightlight in the bathroom instead.
Put your child to bed early—this is the way her body is naturally programmed.
Block early morning sun rays with darkening shades or curtains.

Sleep Cycles, REM Sleep, and Non-REM Sleep

You’ve probably heard of REM (rapid eye movement) and non-REM sleep before. It takes time for children to develop these clearly defined sleep stages. Your child’s “sleep architecture” will be similar to an adult’s at around age 3 or 4 (see below for a description of the adult sleep cycle).

WHILE YOUNG BABIES SLEEP, THEY MOVE THROUGH STAGES DESCRIBED AS

Active (the precursor to REM sleep) in which babies twitch and make sounds and facial expressions.
Quiet (the precursor to non-REM sleep). During quiet sleep, your baby’s breathing is regular and his body is very still. This is a deep sleep. Mature non-REM sleep has distinct stages, but it takes roughly 6 to 8 months for your baby to develop these.

Baby Sleep Is Active

Roughly 50 percent of a newborn’s sleep is active, and little babies can move directly into active sleep (whereas children and adults move through non-REM sleep before entering REM). This is why if your baby falls asleep on your lap, you might see her eyes flutter, mouth grimace, and arms and legs twitch when she does so—she’s more easily roused while in this type of sleep. In adults, muscle movement is inhibited during REM sleep (thankfully, so we do not act out our dreams), but this full motor inhibition takes time to develop, so babies move their arms and legs quite a bit. This active sleep can last for 20 minutes or more before your baby transitions into quiet, or deep sleep. Active sleep (or what is eventually called REM sleep) slowly decreases until roughly age 3, when it makes up 20 percent of sleep (as in adults).

Sleep Cycles Are Shorter

Infant sleep cycles are roughly 60 minutes—it’s not until around school age that your child’s sleep cycles will become 90 minutes, like an adult’s.

Babies Are Noisy Sleepers

Does it sound as if there’s a tiny dinosaur in the room? In active, or REM, sleep, newborns can be quite mobile and vocal. Young babies have a natural tendency to twitch a lot during REM sleep, and they can move their arms and legs, breathe irregularly, and make all kinds of funny sounds. This is sometimes strange or concerning to parents, but it’s normal for your baby to make sounds at night, and it’s important to know that it doesn’t always mean that you need to help her or pick her up. In fact, learning to discern your baby’s sounds at night is really important to her ability to develop self-soothing skills. If your baby snores, gasps, or snorts a lot while sleeping, this is important information for your pediatrician, so he or she can rule out sleep disorders like sleep apnea.

DO BABIES DREAM?

Babies spend a huge amount of time in REM sleep (the stage of sleep when dreaming occurs in older children and adults). Newborns spend about 16 hours asleep each day and roughly 50 percent of that time in REM sleep—that’s a lot of potential dreaming time! Some think that babies do dream, with simple images and experiences (a dog barking, a twirling mobile, a nipple). Others believe that dreaming develops over the years as the brain matures and that, as children grow, their dreams become more complex and rich with thoughts and feelings.

DID YOU KNOW?

Every species sleeps, but we’ve evolved and adapted our own best ways to do so. In a pack of sleeping ducks, the ones on the outside will sleep with only half of their brains so they can keep watch for the rest. Dolphins and whales are also known to put half the brain to sleep at once.

ADULT SLEEP
Adults Cycle Through Four Stages of Sleep

Stage 1. Eyes closed, you’re at the threshold between wakefulness and sleep. You can easily be awakened in this stage—if you are, you may not know you were asleep at all. Sometimes people have the sensation of falling (this is called a “myoclonic jerk,” or a “sleep start”).

Stage 2. This is light sleep, in which your heart rate begins to slow and body temperature begins to go down.

Slow wave sleep. Now you’re in deep sleep. Your blood pressure and heart rate go down and your breathing slows. It’s harder to wake you up. A lot of deep sleep happens in the first part of the night.

REM. Roughly 90 minutes after falling asleep, your brain becomes active, but voluntary muscle movements are inhibited. Brain waves are faster and less organized than in non-REM sleep and the eyes scan back and forth under the lids. REM is the stage in which most dreams occur. As the night progresses, you experience more REM sleep (this is why you’re more likely to dream toward morning and why most kids’ nightmares happen in the second half of the night).