Although our knowledge remains incomplete, a great deal has been discovered about what happens in the brain during sleep: what areas of the brain become active or quiescent, how cellular activity changes, and which neurotransmitters (chemicals that carry signals between nerve cells) are released or blocked. Much has also been learned about the so-called biological clock, a small group of cells that keeps our sleep-wake cycle running on about a twenty-four-hour rhythm. This clock controls not only sleep but also variation in just about every other physiological system throughout the day. The cells that constitute it lie in a primitive area of the brain (the hypothalamus) that also controls many other important automatic functions such as hunger, thirst, temperature, and hormone levels.
Nevertheless, we do not fully understand why we need to sleep, what causes us to sleep, and what purposes sleep serves. There can be no single answer to these questions, in any case, because how you answer them depends on the level at which you approach sleep and waking. At the most basic level, that of neurophysiology, we can say we sleep (and wake) because of changes in the brain’s chemical environment and in its cellular and electrical activity. On a higher level, that of function and behavior, we can say we sleep because sleep serves a restorative function for our bodies and perhaps our minds. Sleep certainly is necessary for us to function properly during the day: if we don’t get enough of it we feel “sleepy,” and this feeling can only be relieved by sleep. Finally, from an evolutionary perspective, the purpose of sleep lies in the benefits it provides for our survival. We can say, as various researchers have, that the purpose of sleep is to protect us from nocturnal predators, to rest the body, to maximize our alertness during the day, or to allow us to process memories. If we turn the whole question around and ask, “Why do we ever stop sleeping and wake up?” we could also answer in terms of physiology (because of chemical and electrical changes in the brain) or of function, behavior, and evolution (we need to be awake to eat, procreate, and care for our young).
Until the 1950s, doctors and other researchers believed that sleep was a single state distinguishable only from waking. However, we now know that sleep itself is divided into two distinctly different states: REM (pronounced as a single word, “rem”), or “rapid-eye-movement” sleep, and non-REM sleep. During non-REM sleep you lie quietly, with a regular heart rate and breathing pattern; it is probably closest to what we usually think of as “sleep,” and it provides most of sleep’s restorative properties. There is very little dreaming in this state, if any, although thoughtlike processes may continue. In REM sleep physiological systems are much more active, and it is in this state that we do nearly all of our dreaming. During the night you cycle back and forth between periods of non-REM and REM sleep as well as having the (usually brief) occasional waking.
After the earliest months of life, non-REM sleep divides further into four distinct stages. These stages range progressively from drowsiness to very deep sleep, and they can be identified in the laboratory by monitoring brain waves, eye movements, and muscle tone.
As you begin to fall asleep, you enter Stage I, the state of drowsiness. Although you are unaware of it, your eyes move about slowly under your closed eyelids. Your awareness of the external world begins to diminish as well. You have no doubt had the experience of becoming drowsy in a lecture or meeting. As you nod off, you miss some of the speaker’s comments, yet you will jerk awake instantly if your name is called or if your head bends so far forward that you are about to fall off the chair. You might think you hadn’t been asleep at all if it weren’t for your lapse of awareness. On waking from this drowsy state, you might remember some thoughts of the kind usually described as “daydreams.” Some people report seeing or hearing things more like the true dreams that occur during REM sleep, except that they are shorter, less well formed, and less bizarre.
If you allow yourself to continue the transition through drowsiness toward deeper sleep, you may notice a sudden jerk of your whole body that actually wakes you briefly and interrupts your descent into sleep. This “hypnagogic startle” is quite normal, although it does not happen every time we fall asleep.
Drowsiness really represents a transitional state between wakefulness and the more fully established stages of non-REM sleep, but we can only identify the arrival of the next stages for certain if we monitor the brain’s electrical activity or “brain waves.” At the onset of Stage II, short bursts of very rapid activity (called “sleep spindles”) and large, slow waves (“K-complexes”) begin to appear (see Figure 2. You can still be awakened easily from this stage, but you may not believe that you had really been asleep, depending on how long you had been in Stage II, how deep into this stage you were at the time of waking, and, as always, on variations between individuals. As in a waking from Stage I, you would probably not report any odd dream images, but you might describe some thoughts or daydreams.
As you fall into still deeper sleep, you enter Stage III and finally Stage IV (similar stages that together can be thought of as your deep sleep). The smaller and faster brain waves of waking and light sleep disappear, replaced predominantly by large, slow “delta” waves. Your breathing and heart rate become very regular, you may sweat profusely, and you will be very difficult to wake. Someone call easily rouse you, as they woulding your name will no longer easily rouse you, as they would from Stage II sleep; instead, you may be oblivious to the sound.
FIGURE 2. BRAIN WAVE PATTERNS IN WAKING AND IN SLEEP
However, if a stimulus is important enough, you will likely wake even from Stage IV. Apparently, even in the deepest sleep our minds can still process some outside information. For example, although it may be difficult to wake you when it is your turn to get up and feed the baby, shouts of “Fire!” or a child’s screams of pain will rouse you promptly. Yet even though you will wake in these emergencies, you will initially be confused. You may be aware that you need to take quick action, but you will have trouble “clearing the cobwebs” from your head so that you can think clearly and sort out what to do. The difficulty one has making the transition from Stage IV sleep to alert waking is very significant in several sleep disorders in children, as you will learn when we discuss sleep terrors, confusional arousals, and sleepwalking (Chapter 13).
In non-REM sleep your muscles are more relaxed than when you are awake. You are able to move (unlike in REM sleep, as we will see), but you lie still because your brain is not sending movement signals to most of your muscles. Disorders such as sleepwalking and sleep-associated head banging are exceptions to this rule.
After one or two periods of non-REM sleep you cycle into REM sleep, a different state entirely. Breathing and heart rate become irregular. Your reflexes, kidney function, and patterns of hormone release change. Your body’s temperature regulation systems are impaired, so you do not sweat or shiver. Males have penile erections in this state; females undergo clitoral engorgement and an increase in vaginal blood flow. The significance of these genital changes is not known.
REM sleep is an active state. Your body uses more oxygen than it does in non-REM sleep, a sign that you are expending more energy. More blood flows to your brain, your brain’s temperature increases, and your brain waves become busy again, resembling a mixture of waking and drowsy patterns. The mind now “wakes up,” but the wakefulness of the dream state is quite unlike that of true waking: you respond mainly to signals originating within your own body instead of those coming from the world about you, and you accept without question the bizarre nature of your dreams.
In this state your muscles have very poor tone, especially in the head and neck, and you become profoundly relaxed. Most nerve impulses that would otherwise pass down the spinal cord and out to the muscles are blocked within the spinal cord, so that not only are your muscles relaxed but much of your body is effectively paralyzed: signals to move may still be sent out from your brain, but they do not reach your muscles. Only the muscles controlling eye movements, respiration, and hearing are spared. Because this blockade is not complete, some of the stronger signals still get through to the muscles, causing frequent small twitches of the hands, legs, or face. So although REM sleep is very active in terms of metabolic and brain function, you remain fairly still.
(In babies, the blockade of motor impulses is not fully developed: more impulses get through to the muscles than in an older child or adult. As a result, a young infant in REM sleep will jerk, grimace, twitch, kick, and even make sounds. Of course, a newborn cannot get up, walk around, and get into trouble. The inhibitory system and the baby mature together, so that by six to twelve months of age—that is, by the time the baby can crawl or walk—most motor impulses are blocked and she stays safely in place.)
The most striking feature of REM sleep is its characteristic bursts of rapid eye movements. During these bursts, the heart rate, blood pressure, respiratory rate, and blood flow to the brain all increase and fluctuate irregularly. If you are awakened during one of these bursts, you will almost certainly report that you were having a dream, and the length of the dream you describe will correspond roughly to the time you had been in that state. Children as young as two have described dreams after such wakings. As for younger children, who lack sufficient language to describe dreams, we cannot know for certain that they do dream or what they dream about. However, since all the other features of REM sleep are in place at birth, it is reasonable to presume that even newborns dream. The first dreams are probably very simple repetitions of daily experiences (sounds, smells, sights); dreams then become more and more complex as higher brain centers and language develop.
We cannot say for sure whether your pattern of eye movements indicates that you were actually “watching” your dream occur. We suspect that this is partly true and that at least some of the muscle twitching corresponds to the actions taking place in the dream. Fortunately, because only a few of the signals to move actually reach your muscles, you merely twitch a little now and then, rather than getting up and moving about, dangerously acting out a dream. One thing this tells us is that sleepwalking and sleep terrors do not result from dreams or nightmares: such complex body movements simply cannot occur during REM sleep.
Some researchers believe that REM sleep has important psychological functions. Their research suggests that REM dreaming allows us to process daytime emotional experiences and transfer recent memories into longer-term storage. Such theories remain unproven. Certainly dreams have emotional significance, but their ultimate importance to the dreamer remains a mystery. REM sleep must serve some purpose, since we all dream every night—even those of us who think we don’t—and if we are deprived of REM sleep for several nights in a row we compensate by getting more REM sleep than usual the next night. Yet, when people are deprived of most of their REM sleep for long periods of time, as a side effect of medication, for example, they don’t seem to show any major ill effects. (Humans cannot be totally deprived of REM sleep, at least not easily. Such studies have been done in animals; complete elimination of REM sleep led to withering and even death.)
Waking a person from REM sleep can be easy or difficult, depending on how important the waking stimulus is to her and how involved she is in her dream. So the clock-radio may not wake you immediately from a really interesting dream; you may even incorporate something you hear on the radio into your dream. On the other hand, an important stimulus such as a burglar alarm will wake you easily and, unlike someone awakened from Stage IV sleep, you will become alert quickly.
To sum up, we seem to live in three distinct states. In the waking state we are rational and we can take care of ourselves and meet our survival needs. In non-REM sleep the body rests and restores itself while the mind rests. And in REM sleep the mind is again active, but it is not rational and it is “disconnected” from the body; major body movements do not take place even though the brain does send out signals to move.
One theory suggests that over the course of evolution REM sleep developed as an intermediate state between non-REM and waking, in which the mind would “wake up” before being “connected” to the body. An animal in non-REM sleep, lying quiet and still except for soft regular breathing, would be relatively safe from predators, but a sudden waking would leave the animal physically active yet confused and disoriented, and thus vulnerable to attack. An animal that first switched into REM sleep would become more alert, but with its brain still disconnected from its muscles it could not make any movement or sound that might alert a predator. Once it was sufficiently alert, the animal could wake fully; the muscle paralysis would disappear, and it could react appropriately to danger. This capacity to check for danger may still be important to humans. We all tend to wake up briefly after an episode of dreaming, and at that moment we are sensitive to anything amiss in our environment: the smell of smoke, footsteps downstairs, or quiet sobbing from the next room. If all seems well, we simply return to sleep, and in the morning we probably won’t remember waking up at all. Young children often cannot return to sleep quickly after these normal arousals because something seems “wrong” to them—in one typical situation, it feels “wrong” that they are alone in their crib instead of in a parent’s arms. This common problem is discussed in detail in Chapter 4.
How Sleep Stages Develop in Children
Sleep patterns begin to develop in babies even before birth. REM sleep appears in the fetus at about six or seven months’ gestation, and non-REM sleep follows a month or so later. In the fetus and infant REM sleep is referred to as “Active Sleep” and non-REM as “Quiet Sleep.” By the end of the eighth month of gestation, both states are well established.
In the newborn, Active Sleep is easy to identify because the baby twitches and breathes irregularly and you can see her eyes dart about under her thin eyelids. Sometimes you may also see her smile briefly. In Quiet Sleep she breathes deeply and lies very still; occasionally you may see fast sucking motions, and now and then a sudden body jerk or “startle.”
Quiet Sleep is still somewhat different from the non-REM sleep of older children and adults. For one thing, it is undifferentiated: the division into separate, distinguishable stages comes later. The brain waves in Quiet Sleep show large slow waves occurring in bursts rather than in a continuous flow. During the first month of life the non-REM brain waves become continuous and startles disappear. By the time a baby is a month old, sleep spindles begin to appear, and over the next month or two we can begin to separate non-REM sleep into lighter and deeper stages. K-complex waves (see Figure 2), characteristic of mature non-REM sleep, do not appear until a baby is about six months old, although precursors appear earlier.
REM sleep is the earliest stage to form. Premature babies spend 80 percent of their sleep time in this state; in full-term infants it makes up half of their sleep. We do not fully understand why REM sleep is so prominent in the early stages of development. We do know that Quiet Sleep requires a certain degree of brain maturation, so one would expect to see less of that stage in newborns. In REM sleep, the higher centers of the brain receive stimulation from deeper, more primitive areas. Impulses come up the same sensory pathways that are used for sight and sound, and perhaps touch, smell, and taste. Later on, such stimuli are probably incorporated by the brain into dream imagery. While we can know nothing of infants’ “dreams,” this state might allow the baby’s developing brain to receive sensory input—to “see” and to “hear”—even before birth. This input might be important to the development of the higher brain centers.
We also know that the baby in the uterus makes no breathing motions in non-REM sleep. If respiratory movements were never practiced, the child would be born with no experience at all in using these muscles that are so vital to survival. However, respiratory motions do occur in REM sleep, and it may be that the baby is also practicing other kinds of motor activity. Muscular impulses in a fetus are not blocked as completely as they are in children and adults, so the fetus has some ability to practice actual body movements in REM sleep. Fortunately for the mother, there is still some blockage of motor impulses during REM sleep, or the baby might never be still!
It may be, then, that REM sleep is most important in the early months as the fetus and baby develops, and progressively less so with increasing age. In fact, although at birth a full-term baby spends half of her sleep time in the REM state, only one-third of her sleep will be REM by age three, and she will reach the adult level of 25 percent by later childhood or adolescence.
Children’s Sleep Cycles
During a period of sleep, children (and adults) cycle back and forth between REM and non-REM sleep. Once non-REM sleep has developed four distinct stages and most of the baby’s sleep time has consolidated into a single nighttime sleep period, the sequence of sleep stages settles into a cyclical pattern that remains fairly constant throughout life. It is important to know something about these sleep cycles if you are to understand the development of normal sleep patterns in your child and the nature of any sleep problems that may occur.
As a child grows from a newborn to an adolescent, the length of the sleep cycle increases from fifty to ninety minutes; also, the total amount of REM sleep and the percentage of sleep time spent in REM decrease until they too reach adult values. The total amount of Stage IV non-REM sleep also decreases throughout childhood and adolescence as total sleep decreases, but it continues to account for about a quarter of the child’s total sleep.
A newborn enters REM sleep immediately after falling asleep. By about three months of age she will enter non-REM first, a pattern that will continue for the rest of her life. Unlike adults, who achieve deep sleep only gradually at the start of the night, young children usually plunge through drowsiness and the lighter stages of non-REM sleep into Stage IV within a few minutes (see Figure 3). In youngsters Stage IV is an extremely deep sleep, and waking a child from that stage may be almost impossible. For example, if your child falls into Stage IV sleep at night in the car, you can probably carry her into the house, change her into her pajamas, and put her in bed with only the slightest sign of movement or arousal. Even a child who always seems to wake when put into her crib after being rocked to sleep will remain asleep as long as she is not placed into the crib until she has reached Stage IV. And if you wake your child from Stage IV sleep to go to the bathroom, she may do so in a semi-awake state and then return to sleep instantly without any recollection of the arousal in the morning. This partial arousal is very similar to what happens during sleepwalking and sleep terrors (see Chapter 13).
A child remains in Stage IV for about an hour or two. After that she undergoes a brief arousal. This partial waking may last for only a few seconds or up to several minutes. Her brain waves change abruptly, showing a mixture of patterns from deep sleep, light sleep, drowsiness, and waking. She will probably move about, perhaps rubbing her face, chewing, turning over, crying a little, or speaking unintelligibly. She may even open her eyes for a moment with a blank stare or sit up briefly before returning to sleep. Occasionally, and briefly, she may even wake fully before the progression of sleep stages continues.
There is actually a spectrum of behaviors that can occur during these arousals. The mild ones just described are quite normal. More dramatic behaviors may also occur at these times, including sleepwalking, sleep terrors, and confused thrashing. These events all occur during partial waking from deep non-REM sleep, and in all of them the child shows features of both sleeping and waking at the same time. We will discuss these behaviors at length in later chapters, but for now remember that they are not stimulated by a dream. As noted above, true dreams, including nightmares, occur only during the REM state.
FIGURE 3. TYPICAL SLEEP STAGE PROGRESSION IN THE YOUNG CHILD
After the brief arousal, there follows a period of a few minutes resembling drowsiness or, perhaps, the beginnings of REM sleep. There may actually be a short REM episode at this point, especially in adolescents and adults, but young children tend to skip it. The first REM episode, whenever it appears, tends to be relatively short—five to ten minutes—and not very intense. There are not many eye movements, and the child’s breathing and heart rates remain fairly stable.
Following this episode of REM or “almost-REM,” the child enters another cycle of deep non-REM sleep. In young children, the descent back into Stage IV sleep will probably be quite rapid, although not as rapid as it was in the first cycle. This period of deep sleep lasts between thirty minutes and two hours (depending, somewhat, on the child’s age), and the arousal that follows it marks the end of the deep sleep period that characterizes the first third of the night. Thus, in children, the first three or four hours of the night are spent mainly in very deep sleep from which the child is not easily roused. Parents are often aware of this fact, because the period of lighter sleep that follows, with more frequent wakings, may well begin at about the time they are going to sleep themselves.
The arousal that ends the initial three or four hours of deep sleep will almost certainly be followed by a REM episode lasting five to twenty minutes. This period of REM sleep may be interrupted by several brief wakings, each followed by a rapid return to sleep. (In babies, REM sleep is particularly unstable, interrupted fairly frequently by brief movements and wakings; however, by the time the baby is around six months old, the inhibition of motor activity is more complete and REM sleep becomes more continuous.)
The REM episode ends with another brief arousal, and the child moves about, adjusts her position, subconsciously checks to see that everything is normal, then goes back to sleep. This momentary awakening has several functions. Changing position is important for the health of skin, muscles, and joints, and the child can check to see that things are the same as when she went to sleep. It is important to be aware that these wakings occur in all children and adults and are quite normal. Parents often perceive them as abnormal, especially if the child cannot return to sleep afterward because the conditions she associates with falling asleep—such as being rocked or patted—are no longer present. This problem of sleep associations will be discussed more fully in Chapter 4.
The middle part of the night, after the first REM episode, begins with another period of non-REM sleep followed by another arousal and a longer and more intense REM episode. Throughout this part of the night, usually lasting about four hours, the child alternates between progressively longer and more intense periods of REM and relatively light periods of non-REM sleep. It is during these hours of light sleep, particularly at the transitions between REM and non-REM sleep, that wakings are most common. Normally these wakings are brief, but it is also at these times that problems of sleeplessness tend to be most severe. Even adolescents and adults may notice that they seem to wake more easily in the middle of the night (to use the bathroom, perhaps) than they do in the morning when their alarm goes off for the day (see Chapter 9). Some children who are kept in bed too long at night sleep only at the beginning and the end, and this middle period of light sleep becomes one of full waking (see Chapter 11).
As morning approaches, children descend once more into fairly deep Stage IV sleep before the final waking; thus the last one or two hours of the night are often quiet, regardless of what came before. Children who tend to wake for the day directly out of this Stage IV period may seem to wake unhappy or crying every morning. The crankiness usually lasts only about ten minutes, and although it may seem worrisome, it is not a cause for concern. Other children, who reenter lighter Stage II or REM sleep before waking, are more likely to wake in good spirits. There is nothing you can do to change the sleep state out of which a child wakes spontaneously.
The very deep sleep that children experience at the end of the night is uncommon at older ages. This isn’t because the capacity for such sleep is lost, but because adults rarely sleep long enough to allow it to emerge. An adult who sleeps eleven hours or so, or an adolescent who sleeps late on the weekend, may show the same end-of-the-night deep sleep typical of young children. A need for deep sleep built up during the day presumably explains why there is deep sleep at the start of the night, but it does not explain why there is more deep sleep at the end. The return to deep sleep near morning is an inherent feature of our sleep cycling system, one controlled by our biological clock (see Chapter 9).
This overview should give you an idea of what is actually happening to your child during the different stages of sleep during the night. It will also help you decide what state your child is in, based on when she fell asleep and what her sleep behavior is like at a given time. These observations will be important as you try to understand what sort of sleep problem your child has and how to deal with it.
Sleep and Waking Patterns
As you read through this section, keep in mind that the sleep requirements described are only approximate and are not the same for all children.
A newborn baby sleeps about sixteen hours a day, but for no more than a few hours at a time. She goes through about seven sleeping and waking periods distributed fairly evenly throughout the day and night. These episodes, which vary from twenty minutes to five or six hours in length, begin with a period of REM sleep, and, depending on the length of the sleep time, they show several REM/non-REM cycles. Even when your baby sleeps well for a few hours, you can usually observe brief arousals. In the early weeks there may seem to be no pattern to the sleep cycle—shorter and longer periods of sleep and waking are scattered over each twenty-four-hour period and vary from day to day.
When the baby is around three months old, daytime sleep settles into a three-nap pattern, with the main naps in the midmorning and midafternoon and, generally, a brief nap in the early evening. (Naps are further discussed below and in Chapter 12.) By the age of three or four months, your baby still sleeps about thirteen hours a day, but now her sleep pattern has consolidated into about four or five regular and predictable sleep periods, with two-thirds of her sleep occurring at night. By now there should be a clear and appropriate differentiation between day and night. At this age most infants have “settled,” which means they are now sleeping through most of the night, at least from a feeding late at night to one in the early morning.
By six months almost all infants have settled, and the periods of continuous nighttime sleep have grown longer. The pattern of settling varies with each child, of course, and your baby’s nighttime wakings may diminish very gradually, or she may settle quickly, as if suddenly forgetting the remaining nighttime feedings. Some babies settle very erratically. In any case, at some point between three and six months, your baby should be sleeping well at night. A typical six-month-old baby sleeps about twelve or thirteen hours total. Nighttime sleep typically lasts about nine and a quarter hours (and as long as eleven hours if daytime naps are short) with only occasional brief wakings. In addition, she will take two one- to two-hour naps each day, one in midmorning and a second in the afternoon. (The evening nap is usually dropped at around this age.) On most schedules, the morning nap will start between 9:30 and 10:30 A.M., and the afternoon nap between 2:00 and 3:00 P.M.
At one year, most children are still sleeping close to twelve hours, with nine or ten of these hours occurring at night. Most youngsters give up their morning nap at this point or within a few months. The single nap that remains generally follows lunch and occurs at 12:30 or 1:00 P.M., between the times where the two naps had previously been. At the transition to a single-nap schedule you may see other, related changes. Since the child now stays awake for longer stretches, sleep may come more easily both at nap time and at night. (This is often particularly noticeable if there were problems before.) Also, although the second nap may have been dropped, total sleep time generally does not change: either the remaining nap or the nighttime sleep lengthens.
By age two, your child should still sleep about nine to ten hours at night, with a one- to two-hour nap after lunch—about eleven and a half hours total. She will probably continue her afternoon nap until at least age three; some children stop by age two, and others continue napping up to age five. The dropping of the final nap is the least predictable of all the developmental changes in napping in the first five years. However, most children stop napping somewhere between their third and fourth birthdays. Those who tend to sleep longer at night, perhaps eleven hours, may simply not be able to sleep during the day. A child who develops this sleep pattern too early may not make it through the day functioning well. Cutting back that child’s nighttime sleep to nine or ten hours may allow the nap to reemerge (see Chapter 12). Some children continue napping when the circumstances are especially conducive to sleep (for example, in the car) or at day care or preschool (where there is strong pressure to lie quietly during a prescribed rest period) but refuse to nap at home or on the weekends.
From age three to adolescence, children need gradually less and less sleep, but the decrease is smaller and far more gradual than was previously thought. Once children are past the toddler age they rarely nap, and they slowly begin to sleep less at night, dropping from about eleven hours in the preschool years to about ten hours in preadolescence. The years from five to twelve years of age are really the most wakeful in a child’s life. Children between those ages generally sleep well at night and “wake well” during the day. The ability to nap in the afternoon is mostly lost (even in siesta cultures); occasional naps may suggest illness. A child who naps most days, especially in school, may have a sleep disorder such as narcolepsy (but, as discussed in Chapter 18, even narcoleptic children at this age may only nap occasionally). Chronic sleep loss at night can cause napping, but that pattern is unusual in the school-age child; instead, the more common consequence of insufficient sleep is behavioral change during the day.
During the four years of puberty rapid changes occur in the child’s body, but the total sleep requirement stays about the same. Children aged fourteen to seventeen still need at least nine hours of sleep for optimal functioning, but few actually get that much, at least on school nights.
Remember, sleep requirements vary among children and families. However, if your child is getting one to two hours more or less sleep than the amount shown for her age in Figure 1, you should at least suspect that her sleep may need adjustment. I hope that the remaining chapters will help you to decide whether she has a problem and, if she does, to identify the cause and correct it.
The Importance of Biological Rhythms
To understand certain childhood sleep problems, we need to look beyond the sleep patterns themselves to the biological systems that control them. The term circadian rhythms refers to biological cycles that repeat about every twenty-four hours. All of us have many such rhythms. They include patterns of sleeping and waking, activity and rest, and hunger and eating, as well as fluctuations in body temperature and hormone release. These cycles must be in harmony if we are to have a sense of well-being during the day. Our ability to fall asleep and stay asleep is closely tied to the timing of these cycles. Typically we fall asleep as our body temperature is dropping toward a daily minimum, and we wake up as it starts rising toward a peak. If we have to wake up at a time when our temperature level is low, we do so only with great difficulty. Similarly, we have trouble falling asleep when our body temperature is still high and has not yet begun to fall.
It is important to know that, in all of us, these cycles are inherently somewhat longer than twenty-four hours. The difference is a matter of only a few minutes. However, exposure to artificial light in the evening, after sunset, and to artificial darkness in our bedrooms in the morning, after sunrise, makes the internal clock run still slower, functioning as if its inherent cycle length were closer to twenty-five hours. We reset the clock’s timing each day through exposure to light in the morning and darkness at night. However, on weekends or vacations, when our schedules are more flexible, many of us begin to operate on this twenty-five-hour day: we go to bed later and get up later. Then we have great difficulty readjusting to the earlier schedule when we return to work or school. This problem, called a sleep phase delay, is discussed in Chapter 10.
Although it is not difficult for us to stay on a twenty-four-hour cycle if we keep to it regularly, problems arise when our routines are irregular or when we try to sleep at times that conflict with our sleep-wake rhythms. Shift workers and adolescents, both of whom tend to sleep different hours on weekdays and weekends, as well as people who travel across time zones, all suffer the malaise, sleep difficulties, and lack of well-being commonly referred to as “jet lag.”
So do children whose sleep cycles have been disrupted or shifted or whose sleep schedules are variable, inconsistent, or otherwise inappropriate. These children may sleep poorly at night, and they may be sleepy or behave badly during the day. It is important to understand this effect, because treatments for problems related to sleep schedule abnormalities are very different from those for other sleep disorders. Besides, even when sleep schedule abnormalities are not the primary problem, their existence often complicates other sleep problems and must be taken into consideration when planning treatment.
The biological rhythms controlling sleep, the ways they can be affected by inappropriate sleep schedules, the various sleep problems that can result, and the ways these sleep problems can be treated are discussed in greater detail in Chapters 9 through 12. For now, remember that normal circadian rhythm function is necessary for normal sleep and optimal daytime function, and that many abnormalities having to do with circadian rhythms are quite simple to identify and correct.