A normal human lifecycle exhibits dramatically different sleep patterns at different points.
Previously, we did a virtual experiment in which we profiled normal sleep in young adults. We did this by doing all-night EEG recordings and then we reduced the resulting ginormous amounts of 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. We all know that babies sleep a lot; we know that young children nap; we know that older children seem to have a sleep/wake switch that is either on or off. They go at a high energy level all day long and then they sleep soundly all night long 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 Granddad or Grandmom, 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 the average about 16 hours a day. That sleep seems to be about equally divided between REM and nonREM sleep. The daily amount of nonREM sleep stays about the same up until adolescence, maybe it increases a little bit in children between one and three years of age, but the amount of REM sleep steadily declines. By the time of puberty, REM sleep makes up 25 to 30% of total sleep time. In the young adult 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. Let’s begin with sleep in infants and very young children up to about five years of age.
The Timing of Sleep
First, the timing of sleep. 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, of course with the exception of cultures where siestas are the norm.
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’s not really a problem, 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 sleep walking. These are things that we will discuss later in the course.
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, the assertion of independence. This is all exacerbated by increased access to social media (telephone, texting, email). Then of course there’s the increased availability of wake-promoting activities such as TV, computer games, and social events, and maybe sometimes—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, I have to build a little background.
Beginning in the first lecture, I made the case that 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 overwhelming and it becomes irresistible.
In the last lecture, I explained the concept of homeostasis and how sleep is really a homeostatically regulated process. We even showed how a characteristic of the EEG, slow-wave activity, has a homeostatic relationship with the duration of prior wake. So, we would expect that if we had a simple test of sleepiness, it would show that sleepiness increases linearly throughout the day. Sound reasonable? Sure does, but that is not the case.
We do have a relatively simple way of assessing sleepiness. It is called the multiple sleep latency test, or the MSLT. The MSLT is very commonly used in sleep clinics to evaluate sleep problems. Here is how an MSLT is done: The person reports to the sleep clinic or the laboratory in the morning, let’s say 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 test. 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 the sleep latency is determined by his accumulated sleep need. The greater the sleep need, 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. So, 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 at 2:00 pm when she had only been awake for six or eight hours. What can explain this paradoxical result?
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 is really showing 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 absolutely 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 all sorts of biochemical, physiological, and behavioral activities of our bodies, including sleep and wake. We’re going to go into considerable detail about circadian rhythms a few lectures from now. But suffice it to say here that one function of our circadian rhythms is “clock-dependent alerting.” The circadian rhythm generates an opponent process that resists the effects of the sleep need that builds up during wakefulness and cause 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. This happens at about 10:30 or 11:00 for me and probably you can pinpoint the time in the evening when your clock-dependent alerting goes offline.
What does all of this have to do with the rebellious, contrary, stubborn adolescent who won’t go to bed at a reasonable hour and who has to be pulled out of bed in the morning to get to school before the late bell rings? 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. You are surely 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 turn 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?