Bio Rhythms and Sleep - 10.3

How much sleep and what kind of sleep we get changes across our lifetime. As infants, we sleep a lot; as we grow, we sleep less and less until we hit adolescence, when once again sleep seems precious. After that, we sleep less as we age, sometimes to our disappointment. These changes as we grow up and grow old suggest that the function(s) of sleep are more important during some stages of life than others.

Human infants sleep a lot, but a clear cycle of sleeping and waking takes several weeks (that feel like years to parents) to settle in (FIGURE 10.14). A 24-hour rhythm is generally evident by 16 weeks of age. Infant sleep is characterized by shorter sleep cycles than those of adults, probably reflecting the relative immaturity of the brain, since sleep cycles in prematurely born infants are even shorter than in full-term newborns.

FIGURE 10.14 The Trouble with Babies This classic study may represent an extreme example of a baby slow to entrain to the day-night rhythm. View larger image

Infant mammals also show a large percentage of REM sleep. In humans, for example, half of sleep in the first 2 weeks of life is REM sleep. The prominence of REM sleep is even greater in premature infants. Unlike most adults, human infants can move directly from an awake state to REM sleep. The REM sleep of infants is quite active, accompanied by muscle twitching, smiles, grimaces, and vocalizations. The preponderance of REM sleep early in life (FIGURE 10.15) suggests that this state provides stimulation that is essential to maturation of the nervous system. By contrast, orcas and bottlenose dolphins appear to spend little or no time in REM sleep (or any other sleep stage) for the first month of life (Lyamin and Siegel, 2019), presumably because they have to surface often to breathe. So, either REM sleep does not fill a crucial need in all mammalian infants, or dolphin and whale infants have evolved an alternative way to fill that need.

FIGURE 10.15 Human Sleep Patterns Change with Age View larger image

Most people sleep appreciably less as they age

The character of sleep changes in old age, though more slowly than in early development. FIGURE 10.16 shows the sleep pattern typical of an elderly person. The total amount of sleep declines, while the number of awakenings increases (compare with Figure 10.12). Lack of sleep, or insomnia (which we discuss at the end of this chapter), is a common complaint of the elderly and is associated with a variety of physical and cognitive impairments (Winer et al., 2021).

FIGURE 10.16 The Typical Pattern of Sleep in an Elderly Person Compare this recording with the young adult sleep pattern shown in Figure 10.12. View larger image

I Need Sleep! Doing without sleep has one clear effect: you feel sleepy. View larger image

In humans and other mammals, the most dramatic decline is in stage 3 sleep; 60-year-old people spend only about half as much time in stage 3 as they did at age 20 (Mander et al., 2017). By 90 years of age, stage 3 sleep has disappeared. This decline in stage 3 sleep may be related to diminished cognitive functioning, since an especially marked reduction of stage 3 SWS characterizes the sleep of people with senile dementia. Growth hormone is secreted primarily during stage 3 SWS (see Table 10.1), so perhaps the loss of growth hormone due to disrupted sleep in the elderly leads to the cognitive deficits (Stitch et al., 2022). Loss of SWS probably also impairs memory processes (discussed below) in older people and people with dementia (Winer et al., 2021).

Most elderly people fall asleep easily enough, but then they may have a hard time staying asleep, which causes sleep “dissatisfaction.” As in so many things, attitude may be important for how you experience sleep loss as you age. Objective measures of sleep suggest that elderly people who complain of poor sleep may actually sleep more than those who are satisfied with their sleep (McCrae et al., 2005). Perhaps if, as you grow older, you can regard waking up at 3:00 am as a “bonus” (a little more time awake before you die), you will be more satisfied with the sleep you get.

Manipulating sleep reveals an underlying structure

Another persuasive clue that sleep is important is revealed when we go without it. First of all, our mental function is impaired. This is bad news for college students, who rarely get enough sleep, just when they’re supposed to be learning how to make their way in the world. In addition, after sleep deprivation we tend to sleep more than we would have, as though catching up on something we need, as we’ll see.

Most of us at one time or another have been willing or not-so-willing participants in informal sleep deprivation experiments. Thus, most of us are aware of the primary effect of partial or total sleep deprivation: it makes us sleepy. It has other effects as well. Early reports from sleep deprivation studies emphasized a similarity between schizophrenia and “bizarre” behavior provoked by sleep deprivation. But examination of people with schizophrenia does not fit this view. For example, many people with schizophrenia show sleep-waking cycles similar to those of typical adults, and sleep deprivation does not exacerbate their symptoms.

The behavioral effects of prolonged, total sleep deprivation vary appreciably and may depend on some general personality factors and on age. In studies employing prolonged total deprivation—205 hours (8.5 days!)—a few participants showed occasional episodes of hallucinations. But the most common behavior changes were increases in irritability, difficulty in concentrating, and episodes of disorientation.

You don’t need to resort to total sleep deprivation to see effects. Moderate effects of sleep debt can accumulate with successive nights of little sleep. In one study, research participants who got 6 or 4 hours of sleep per night for 2 weeks showed ever-mounting deficits in attention tasks and in speed of reaction, compared with those sleeping 8 hours per night (Van Dongen et al., 2003). By the end of the study, the people getting less than 8 hours of sleep per night had cognitive deficits equivalent to those of participants who had been totally sleep-deprived for 3 days! If that doesn’t persuade you about the importance of sleep for cognition, note that the GPAs of first-year college students are positively correlated with how much they sleep at night (Creswell et al., 2023)!

Sleep recovery may take time

One of the most famous cases of sleep deprivation began as a high school student’s science project. Researchers became involved only after Randy Gardner had started his deprivation schedule, which is why we have no data about his sleep before he