A normal human lifecycle exhibits dramatically different sleep patterns at different points.
To understand the variation across the lifespan, let’s use a real-life example. In a small virtual experiment, my colleagues and I profiled normal sleep in young adults by doing all-night EEG recordings; then, we reduced the enormous amounts of resulting data into hypnograms. The common features of those hypnograms characterized the sleep of healthy young adults. We saw that undisturbed sleep lasted about eight hours. Sleep began with a rapid progression through nonREM Stages 1 and 2 to nonREM Stage 3, the deep slow-wave sleep. The first bout of nonREM sleep lasted over an hour. NonREM sleep then lightened through Stage 2 and maybe even to Stage 1 before transitioning into the first bout of REM sleep. This first bout of REM sleep lasted only about 15 to 20 minutes. This cycle between nonREM and REM sleep continued four more times through the night with a periodicity of about 90 minutes.
Most of the Stage 3 nonREM sleep was in the first two cycles and most of the REM sleep was in the last three cycles. The total amount of REM sleep was about 20 to 25% of the total sleep time. Brief awakenings occurred occasionally in the second half of the night, but they don’t last long. These results give us a place to start in asking how sleep changes across the life span. Common knowledge tells us that babies sleep a lot; we know that young children nap, and that older children seem to have a sleep/wake switch that is either on or off.
This is a transcript from the video series Secrets of Sleep Science: From Dreams to Disorders. Watch it now, on The Great Courses Plus.
Children go at a high energy level all day long and then they sleep soundly all night until they spring out of bed in the morning. In contrast, it practically takes a tow truck to get many adolescents out of bed in the morning. Then we have our grandparents, dozing in a recliner chair while trying to read or watch daytime TV. They go to bed early, but in the morning we hear complaints that they just can’t get a good night’s sleep. Clearly, there are major changes in sleep across the life span. Are there biological bases for these changes, and what are their consequences?
In the first weeks of life, the newborn sleeps on the average about 16 hours a day.
The most general description of sleep across the life span is the amount of sleep a healthy person gets at different ages. In the first weeks of life, the newborn sleeps on average about 16 hours a day. That sleep seems to be about equally divided between REM and nonREM sleep.
Learn more about the tools researchers use to study sleep patterns
The daily amount of nonREM sleep stays about the same up till adolescence, and perhaps it increases a little bit in children between one and three years of age. But the amount of REM sleep for children steadily declines. By the time of puberty, REM sleep makes up 25 to 30% of total sleep time. In young adults, it is down to 20 to 25% of total sleep time. Throughout adulthood and into old age, the total daily sleep time gradually decreases, but the percent of REM and nonREM remains about the same.
Six hours of sleep per night is common for the elderly. There are more changes in sleep across the life span than are reflected in total sleep amounts. To understand lifespan development, let’s start with infants and very young children up to about five years of age.
The Timing of Sleep
A hallmark of development that is a huge relief for parents is the first time the infant sleeps through the night.
The timing of sleep is astonishingly important to our health and wellness, especially for infants. Sleep in the newborn is distributed around the clock, creating some serious sleep disruption and deprivation for the parents. By about two to four months of age, the infant shows a tendency to sleep more at night than during the day, and will occasionally sleep for a continuous five or six hours. A hallmark of development that is a huge relief for parents is the first time the infant sleeps through the night. About 60% of six-month-old babies sleep through the night, and by nine months of age, it is 80%.
Of course, infants continue to have sleep bouts during the day. By six months of age, this is usually down to two daytime naps. By the time they are a year to a year-and-a-half old, naps are down to one a day. The pattern of a daily nap usually continues until about five years of age when they go to kindergarten. After that, the common human pattern of one consolidated sleep phase per day predominates throughout life, except for cultures where siestas are the norm.
Learn more about how your brain’s circadian rhythms regulate sleep
There are a few common sleep problems that parents of young children encounter, but they’re not serious. Probably the most common is bedtime battles. Children are generally very active during the day and may find it difficult to wind down in the evening. A regular bedtime schedule along with calming activities such as a bath, storytelling, and reading usually solves this problem.
Another common behavior that can be problematic is when a one- to two-year-old figures out how to get out of the crib and insists on crawling into bed with mommy and daddy. This can be a difficult habit to break once it gets started. Two- to five-year-olds can have problems with nightmares and sleepwalking.
Probably the most critical change in the timing of sleep occurs at puberty. There is a strong tendency for adolescents to move towards a later bedtime. Undoubtedly psychosocial factors are at play here—the assertion of maturity and the assertion of independence.
This is all exacerbated by increased access to social media (telephone, texting, and email). Then, there’s the increased availability of wake-promoting activities such as TV, computer games, and social events, and—maybe sometimes—school work. What has only recently been established experimentally is a biological cause of the delayed sleep time that occurs at the time of puberty. To explain this, let’s build a little background.
Learn more about the neural systems that control sleep and wakefulness
Sleep Need and Circadian Rhythms
Sleep need accumulates during wakefulness and this sleep need is discharged during sleep. The accumulation of sleep need results in sleepiness, or a drive to go to sleep that becomes so overwhelming it becomes irresistible.
Sleep is a homeostatically regulated process. Even a characteristic of the EEG, slow-wave activity, has a homeostatic relationship with the duration of prior wake. Reasonably, we would expect that if we had a simple test of sleepiness, it would show that sleepiness increases linearly throughout the day. But that is not the case.
Researchers do have a relatively simple way of assessing sleepiness. It is called the multiple sleep latency test, or the MSLT. The MSLT is commonly used in sleep clinics to evaluate sleep problems. To conduct an MSLT, the person reports to the sleep clinic or the laboratory in the morning, fo example, around 8:00 am. He is outfitted with EEG electrodes. Then, at regular intervals throughout the day, for example, 10:00 am, noon, 2:00 pm, 4:00 pm, 6:00 pm, and 8:00 pm, he is taken into a quiet, dark bedroom and put into a comfy bed.
The question is, how long it will take him to fall asleep? How long it takes to fall asleep is sleep latency, hence multiple sleep latency tests. As soon as he falls asleep, he is woken up, taken out of the bedroom, and engaged in quiet wakeful activities until the next test time. The assumption is that sleep latency is determined by his accumulated sleep need. The greater the sleep needs, the shorter the sleep latency.
For a person who had a good night’s sleep the night before, her sleep latency in the first test at 10:00 am will be about 20 minutes. This sleep latency will gradually decrease, through to the 2:00 pm test when it might be only 10 minutes. The surprise is that the sleep latency at the 6:00 pm and 8:00 pm tests is almost as high as it was at 10:00 am. Thus, the test tells us that the individual is less sleepy in the early evening after being awake for 12 hours than she was at noon or 2:00 pm when she had only been awake for six or eight hours. What can explain this paradoxical result?
Learn more about the three basic characteristics of sleep
One commonly held idea about the midday slump is that it is due to eating and digesting lunch. This is not the case. The MSLT is the same whether you eat lunch or not. The typical protocol for an MSLT is to give small snacks of equal caloric value at regular intervals throughout the day.
What the MSLT shows is a mechanism or a process in the brain that opposes sleepiness. This process involves the ability of our brains to generate internally a biological rhythm called a circadian rhythm. Circadian rhythms are common to virtually all organisms on Earth. They are characterized by the fact that even under constant environmental conditions, they have a period that is about a day, hence the term “circadian.” Circa for about and dia for day.
Our circadian rhythms modulate the many biochemical, physiological, and behavioral activities of our bodies, including sleep and wake. Suffice it to say, one function of our circadian rhythms is “clock-dependent alerting.”
The circadian rhythm generates an opponent process that resists the effects of sleep need that builds during wakefulness and causes sleepiness. In the evening, when we have accumulated the most sleep need, clock-dependent alerting pushes back on the need to sleep and sustains alertness. When this clock-dependent alerting fades, sleep need takes over and it becomes very difficult to stay awake. Personally, this happens at about 10:30 or 11:00 for me; you can probably pinpoint the time in the evening when your clock-dependent alerting goes offline.
Changing Sleep in Adolescence
The change in the phase relationships between the opponent processes essentially turns prepubertal larks into adolescent owls.
It may seem challenging to make the connection between the rebellious, contrary, stubborn adolescent who won’t go to bed at a reasonable hour and the system for our circadian rhythms and clock-dependent alerting. Extensive sleep studies led by Dr. Mary Carskadon at Brown University have shown that at the time of puberty, the phase relationships between the two opponent processes, sleep need and clock-dependent alerting, change. All of us are familiar with people who are larks, early to bed, early to rise, and with people who are owls, late to bed, late to rise. The change in the phase relationships between the opponent processes essentially turns prepubertal larks into adolescent owls. As a result, they cannot fall asleep easily at what used to be their customary bedtime, and they cannot wake up fully alert at the early morning hour necessary to get washed, get dressed, eat breakfast, and not miss the school bus. Because of this change in phase relationships of the opponent processes, many adolescents accumulate considerable sleep debt. Such sleep debt has strong negative consequences for mood, cognitive performance, and health.
In light of these new facts, it would seem wise to rethink what is almost a universal organizational pattern in our schools. Given limited transportation facilities, it’s common practice to bus the junior and senior high school students early in the morning and the elementary grade kids later. Shouldn’t it be the other way around?
Learn more about the cellular function of non-REM sleep
Common Questions About Sleep Patterns
How much sleep is required depends on age and activity. The average young adult needs around eight hours; however, the need for sleep decreases as you age. Many people report feeling fine after four hours of sleep.
The body generally goes through five stages of sleep. The first is a repeating cycle, next, a transitional NREM phase, and then a light stage of NREM relaxation. The following two phases are the most important phases of NREM sleep that rejuvenate a person, called the slow-wave stage, and finally REM sleep, where dreaming takes place.