The average characteristics of sleep described up to now generally refer to those seen at the beginning of the night in adults, especially young adulthood (20-40 years). As described below, these characteristics tend to change over the course of one night, as well as over the lifespan (birth to old age).

Changes In Sleep During the Night

Over the course of one night, we cycle several times through NREM and REM sleep stages. One cycle of sleep is shown in Figure 1.

Figure 1. One cycle of sleep

The first sleep cycle of the night tends to last only about 60-75 minutes, and it's possible that we may wake up briefly from light sleep at the end of this cycle. Afterwards, each sleep cycle lasts about 90 minutes, on average. In fact, because we're most likely to wake up from light sleep, you may notice that, when you awaken from sleep early in the night, you're most likely to do so about 60-90 minutes or 2.5-3.0 hours (60-90 minutes + 90 minutes) after you first fall asleep. Because most young adults sleep from seven to nine hours, we go through about five to six sleep cycles per night. As the night progresses, however, two major changes occur in the lengths of the stages making up each cycle:

(a) The duration of SW sleep decreases in each successive cycle until Stages 3 and 4 virtually disappear during the second half of the night.
(b) The duration of REM sleep increases in each successive cycle, lasting only about 1-5 minutes during the first cycle and about 40 minutes during the last cycle just before waking up.

In short, as the length of each slow-wave episode decreases, the length of each REM episode increases, thereby keeping the length of the entire cycle at about 90 minutes throughout the night. Figure 2 illustrates these changes.

Figure 2. The cycling of sleep stages during an eight-hour night
(from Fernald, 1997, p. 166)

PUT IN CIRCADIAN RHYTHMS AND ITS LINK WITH THE TIMING OF SLEEP.

Changes In Sleep Over the Lifespan

From birth to old age, there are many changes in the general characteristics of sleep. Age groups differ in the patterning and timing of individual sleep stages, as well as in the total amount of time spent sleeping. Figure 3 shows average changes in REM sleep (light gray) and NREM sleep (dark gray below the light gray) sleep over the life span. (Please keep in mind that this graph illustrates differences in the means of age groups. It ignores the broad range of differences among individuals within each age group.)

Figure 3. Developmental changes in the proportions of REM and NREM from birth to 90 Years
(from Peterson, 1997, p. 195)

Newborns. Newborns sleep about 16 hours per day, on average, with about six short sleep periods spread equally over 24 hours. During this early part of life, sleep is divided into only two stages: active sleep and quiet sleep. When newborns first fall to sleep, they enter active sleep, during which they show a great deal of bodily activity such as head movements, rapid eye-movements, fast and irregular respiration, and increased heart rate. Their EEG recordings show brain activity similar to that seen during REM sleep in adults. Active sleep, therefore, seems to be the newborn’s version of REM sleep. After about 30 minutes of active sleep, newborns enter quiet sleep, a stage in which they show few movements except occasional jerks of their bodies (that look to me as if they have just been startled). Their EEG recordings show brain activity similar to a combination of the four stages of NREM sleep in adults. Quiet sleep, therefore, seems to be the newborn’s version of NREM sleep.

The sleep of newborns shows three major differences from the sleep of adults:

(a) The newborn cycles through the quiet and active stages about once every 60 minutes instead of the 90-minute cycles seen in adults.
(b) The newborn enters sleep by going into a REM-like stage instead of a NREM stage.
(c) The REM-like stage of active sleep makes up about 50 percent of each sleep period instead of the approximately 15-20 percent seen in adults.

It takes about 2 years for the sleep of babies to develop fully into a pattern similar to that seen in adults.

Older infants. Over the first year of life, the total amount of daily sleep decreases to about 12-13 hours per day. In addition, the several periods of sleep seen in the newborn begin to combine into fewer and longer sleep periods until, by about six months of age, most of the infant's sleeping occurs during the night, with two much shorter periods of sleep occurring during the day. Within a few months after birth, characteristics of the separate NREM sleep stages begin to emerge. As one example of this, sleep spindles, which define Stage 2 in older individuals, are first observed during the second month after birth and continue to develop and become more distinct over the next several months.

Toddlers and preschoolers. The amount and rate of change in sleep stages from about 1 to 5 years of age varies widely from one individual to another, just as with any other physical or mental characteristic. Nevertheless, some average patterns can be described. By 2 years of age, the four NREM sleep stages and REM sleep become fully distinct, although the duration of each differs from that described previously for young adults. The total amount of sleep per 24-hour period declines from about 13 hours at 1 year to about 11 hours at 5 years. Most of this decrease occurs because of reductions in daytime napping: by 5 years, less than half of children take naps regularly.

But perhaps the biggest change during this time occurs in the amount of REM sleep. At birth, active sleep (the REM-like stage of newborns) makes up about 50% of the entire sleep period. But by 2 years of age, REM sleep makes up only about 25-30% of sleep. Somewhere between 3 and 5 years of age, REM sleep decreases to only about 20% of the sleep period — a proportion that changes little after this time. That is, older children, adolescents, young adults, middle-aged adults, and elderly adults all spend about 15-20% of their sleep in REM. Nonetheless, you should keep in mind that, although the proportion of REM sleep does not change after about 5 years of age, the absolute amount of time spent in REM will decrease after this age because, as we get older, we sleep fewer hours (see Figure 3 above).

Beginning sometime between 3 and 5 years of age, sleep disturbances — such as sleepwalking, nightmares, and sleep terrors — develop in many children. For example, probably all children between the ages of 3 and 5 years have nightmares at least occasionally, and many have frequent nightmares (nightmares that occur at least once per week; Hartmann, 1994). Although these disturbances can become a cause of concern for parents, they usually decrease over time until, by about 11-12 years, few children experience them frequently.

Grade school through adolescence. Nightmares continue throughout grade school (6-11 years) but become infrequent by early adolescence (13 years), and are rare by the beginning of adulthood (20 years). Probably about 50% of grade-school children have frequent nightmares, especially when younger, whereas only about 5% of young adults have frequent nightmares. The total amount of sleep decreases throughout grade school and adolescence, from about 10 hours per night at 6 years to about 8 hours per night at 17 years. The amount of SW sleep decreases dramatically after about 12 years — a 40% decrease by late adolescence.

Adolescents tend to get less sleep than they need. Many set their own bed times, which tend to be too late at night given that they still need to get up early for school and other activities. Furthermore the circadian rhythms of teenagers seem to be shifted compared to those of adults: the normal biological rhythms of teens cause them to fall asleep later at night and to wake up later in the morning compared to adults. Thus, the combination of going to bed later at night but needing to wake up early in the morning for daily activities leads to chronic sleep deficits in many teenagers. Because of this, many adolescents show an increase in daytime napping in an attempt to make up for their decreased nighttime sleep.

Adulthood. From early to late adulthood (from about 20 to 65 years), there are large changes in the duration of particular stages of sleep. Table 1 shows some of these changes by comparing the proportion of the total sleep period spent in each stage of sleep in young and elderly adults.

Table 1. The percentage of the sleep period spent in each stage of sleep in younger and older adults
(From Webb, 1975)

Three major differences between the two age groups can be observed (see the last row in Table 1):

(a) Awake. There is a large increase from early to late adulthood in the amount of time spent awake during the night.
(b) Light Sleep (Stages 1-2). There is a large increase from early to late adulthood in the amount of time spent in light sleep.
(c) Deep Sleep (Stages 3-4). There is a large decrease from early to late adulthood in the amount of time spent in deep sleep.
(d) REM. There is little change from early to late adulthood in the amount of time spent in REM sleep.

By about 60 years of age, many people no longer are getting any SW sleep — a finding most likely to be observed in elderly males. Older adults also tend to take more daytime naps, presumably to make up for the reduced amounts of nighttime sleep. In general, the sleep of women changes less dramatically with age than does the sleep of men. Nevertheless, it is very difficult to generalize because, during the later years of life, there is an increase in the amount of variance observed in sleep patterns.

There are interesting changes in dreaming that occur in some people during adulthood. In late adulthood, people sometimes report that they dream less and recall fewer dreams than when they were younger. In fact, when an elderly person develops a progressive brain disease, such as Alzheimer's Disorder, the amount of REM sleep tends to decline dramatically (Montplaisir, Petit, Gauthier, Gaudreau, & Décary, 1998). About 50% of adults report that they never experience nightmares. On the other hand, nightmares sometimes become more frequent in older adults who are taking medicine for Parkinson’s disorder or hypertension, probably because of the effects of the medications on brain activity. In addition, adults who experience severely distressing events in their lives also are more likely to report frequent nightmares. In rare instances, adults are lifelong sufferers of frequent nightmares, whether they are under stress or not, and whether they are taking medications or not. Surprisingly, the majority of such people report that they have no desire to end their frequent nightmares: they either make use of the nightmares (especially in artistic work) or feel as if their nightmares are an important part of who they are.

Study Questions for Section 2-9

1. How would you define one cycle of sleep?
2. How many cycles of sleep would an adult go though if she slept a little over four hours during the night?
3. Are people more likely to have a nightmare during the first or the second half of the night?
4. Are people more likely to have a sleep terror during the first or the second half of the night?
5. Are people more likely to sleepwalk during the first or the second half of the night?
6. What happens to the amount of SW sleep as the night progresses?
7. What happens to the amount of NREM sleep as the night progresses?
8. What happens to the amount of REM sleep as the night progresses?
9. What are the main characteristics of the two stages of sleep in newborns, and how do they compare to the stages of REM and NREM sleep in adults?
10. In what ways do the onset of sleep and the timing of the sleep cycle differ between newborns and adults?
11. By about what age have the five distinct stages of sleep developed fully?
12. At what point during the life span do nightmares, sleepwalking, and sleep terrors first develop, and when are they usually no longer a problem?
13. At about what age does the percentage of REM sleep per night reach the level seen in adults?
14. At about what age do we observe the first large decrease in the amount of SW sleep obtained per night?
15. Which stages of sleep show the largest changes in duration from early to late adulthood? (Note: Please describe these changes in your answer.)
16. What effects do progressive brain diseases sometimes have on the amount of REM sleep obtained per night?

Go to Quiz 2-9 questions

Go to Readings Section 2-10