Learning Objectives
By the end of this section, you will be able to:
- Explain the physiology of sleep
- Identify the stages of sleep
- Describe the sleep patterns and needs throughout the life span
Sleep is a necessary part of life with potentially significant, and even dangerous, repercussions when individuals do not receive adequate amounts and quality. The physiological processes involved in and during sleep are not simple. To an observer, however, sleep appears to be a simple enough set of actions:
- fatigue and sleepiness
- falling asleep
- maintaining sleep
- cycling through stages of sleep
- waking rejuvenated and ready for the waking, active hours
This section explores the physiology involved in sleep, starting with the contributions of the reticular activating system (RAS)—a group of neurons that control sleep and wakefulness, arousal, and consciousness—and the circadian rhythms, or the biological rhythm associated with metabolism, temperature, sleep, and wakefulness. As nurses, it is important to understand the important role sleep plays in people’s lives, including contributions to health, illness, and daily function. This section will examine the physiology and stages of sleep and then delve into sleep patterns and the changing sleep needs throughout the life span.
Physiology of Sleep
Sleep, like respiration, is a basic physiological function, under neurological control and originating in the brain stem. There are a variety of processes that occur while sleeping, including storing and cataloguing new information, and changing short-term memories into long-term memories (Nunez & Lamoreux, 2023). The glymphatic system is involved in regulation of interstitial fluid movement and subsequent removal of waste from the brain and cerebrospinal fluid (CSF) and is directed by sleep and circadian cycles (Hablitz & Nedergaard, 2021; Nunez & Lamoreux, 2023). Potassium and lactic acid are two of the waste products removed by processes of the glymphatic system. A peptide implicated in development of Alzheimer disease called beta amyloid, and tau, a protein also associated with Alzheimer disease development, are other wastes eliminated by the glymphatic system (Hablitz & Nedergaard, 2021). Certain areas of the brain, such as the amygdala and hippocampus related to emotional regulation, are more active during sleep (Nunez & Lamoreux, 2023).
The body requires an adequate amount of sleep. Adequacy of sleep changes over the human life span, ranging from sixteen to eighteen hours per day for newborns, to seven to nine hours of the day for adults (see more about developmental stages and sleep later in a later section of the chapter). Achieving enough sleep for one’s stage of development and age allows people to grow, function, and thrive throughout life. Cognitive actions, such as learning, problem-solving, concentration, and creativity, are performed best when the body and brain have had adequate sleep. Sleep deprivation inhibits positive adaptations by the brain to stress, resulting in less effective coping responses and negative reactions while awake. Examples of such maladaptive responses include eating disorders, and use of caffeine, alcohol, and other substances. Proper organ function relies on adequate sleep as do healthy immune systems, use of insulin, effective cognition, and cardiac function.
Sleep involves coordination of the properties of homeostasis and circadian rhythm. A person tends to associate feelings of fatigue and drowsiness with a need for sleep, which is the homeostatic mechanisms at work. The circadian rhythm includes the body’s natural responses to light and dark, which normally involve being awake and alert during hours of light, and feeling tired, sleepy, and subsequently sleeping, during periods of darkness. Melatonin is a modulator involved in circadian rhythm changes, with increasing levels during darkness, and decreasing levels during hours of light.
During hours of sleep, consciousness remains active, but is generally reduced, with a change in responsiveness. When a person is awake, they are acutely aware of external stimuli; while asleep, consciousness is predominantly responsive to internal rather than external stimuli (Brinkman et al., 2023). As a person experiences sleepiness, if there is a safe environment for sleep, the hypothalamus initiates actions to inhibit areas of the brain involved in arousal. As the person reaches the sleep state, the regions of the brain stem, thalamus, and cortex operationally disengage (Brinkman et al., 2023). An electroencephalogram (EEG), a linear tracing of the electricity of brain waves, is able to identify the different periods or stages of sleep. In brief, the first period is non-rapid eye movement (NREM) sleep, and is comprised of three stages of sleep itself (Figure 28.2). The EEG waveforms change during the different stages. As one falls asleep (stage I sleep), a fast alpha wave is typical while awake and resting, but is followed by a theta wave, which is also fast, with variable frequencies (Patel et al., 2024). In stage II, two characteristic EEG changes are apparent: a short, intermittent burst of neurological activity called sleep spindle and K-complex, each of which lasts about one second and is associated with the delta wave, which is slower but have more amplitude. In stage III of NREM sleep, the EEG displays slow delta waves; this stage is also referred to as slow-wave sleep (SWS) (Patel et al., 2024).
The other sleep stage is rapid eye movement (REM) sleep, which is associated with fast movements of both eyes. REM is generated by activation of “REM-on neurons” that stimulate responses from both the parasympathetic and sympathetic nervous systems (Brinkman et al., 2023). This increased brain activity is apparent on the EEG, as the waveform during REM sleep appears much like that of someone who is awake (Patel et al., 2024).
Reticular Activating System
Located in the anterior of the brain stem, the reticular activating system (RAS) is integral in some of the most basic functions, including consciousness, sensory perceptions and responses, ability to focus, and the sleep-wake cycle. Specifically, the RAS adjusts fast and slow sleep rhythms, which assist with coordination of wakefulness and sleep (Arguinchona & Tadi, 2023). Additionally, the RAS moderates muscle tone during different stages of sleep, most particularly contributing to the lack of muscle tone apparent during REM sleep, which prevents movement of the extremities during dreams.
The RAS is also involved in responses to pain and some mental health and neurological disorders, such as schizophrenia, Parkinson disease, post-traumatic stress disorder (PTSD), and narcolepsy, a sleep disorder involving sleep and wakefulness with abnormal episodes of daytime sleep (see Central Disorders of Hypersomnolence).
Link to Learning
Watch this RAS video for more information about the RAS.
Circadian Rhythms
Wakefulness, metabolism, temperature, and the timing of sleepiness and subsequent sleep, are part of circadian rhythms. Such rhythms likely originated historically from humans as hunters and gatherers (Casiraghi et al., 2021). These early peoples, by necessity, would use the improved visibility during daylight hours for most hunting, searching for other food sources, and other activities. Their own safety was also of paramount importance, and therefore it was also valuable to be able to see potential dangers.
The control of circadian rhythms initiate within the hypothalamus, where there are centers for sleep and wakefulness. These are set to respond to light and darkness through messages from the retinas. The results include feeling sleepy, physiologically welcoming the onset of sleep two or three hours after dark, and naturally awakening shortly before, or at sunrise (Figure 28.3).
Chemical contributions to sleep are regulated by the circadian rhythm and include the nighttime secretion of melatonin, which assists in promoting sleep by opening the “sleep gate” (Viganò et al., 2023), promoting NREM sleep and improving the continuity of REM sleep. The release of prolactin, adrenocorticotropic hormone (ACTH), and norepinephrine are also under circadian influence (Patel et al., 2024). The neurotransmitter adenosine is associated with a person’s feeling the need for sleep (Bryan, 2023). Additionally, adenosine is thought to promote deep slow-wave sleep and its restorative properties (Bryan, 2023). This chemical is metabolized during sleep, so low levels are apparent in the morning. Cortisol is another chemical involved in circadian rhythm, as its early-morning release prepares the body to awaken.
Stages of Sleep
Sleep occurs in a series of four stages, briefly introduced earlier, which accord with associated EEG waves. These sleep stages happen throughout the sleep session (typically overnight) in a cyclical pattern, with a person spending about 75 percent of sleep time in NREM stages I through III, and 25 percent in stage IV REM sleep (Suni, 2023). Each cycle encompasses approximately 90 to 110 minutes, and it is normal to experience four or five cycles during a night’s sleep.
Stage I
Stage I (N1) is the first stage of sleep onset; this stage normally happens quickly—on average taking no more than seven minutes of body and brain relaxation to fall asleep (Patel et al., 2024; Suni, 2023). Only about five percent of total sleep time is spent in stage I (Patel et al., 2024). As the individual relaxes, physiological activities, including those in the brain, become slower. This is often noted by a reduced heart rate, slow and regular respirations, and slow, rolling eye movements (Lockett, 2023). There may be occasional muscular movements seen, because skeletal muscle tone remains present during this phase (Suni, 2023). It is during this stage that EEG waves change from the beta waves associated with being awake to alpha waves of a relaxed but awake state, to theta waves of light sleep (Patel et al., 2024). If not disturbed during stage I, a person usually proceeds promptly to stage II and may not return to this stage for any significant amount of time. It is easy to awaken someone from stage I sleep.
Stage II
Stage II (N2) sleep involves more changes, as sleep becomes deeper, with heart rate, respiratory rate and temperature all continuing to decrease. Eye movement that slowed during stage I stops and people may grind teeth, with brief episodes of brain activity that offer resistance to wakening (Suni, 2023). Such bursts of activity within the brain are presumed to be involved in the solidification of memories (Patel et al., 2024). It is during this cycle that delta waves begin, with sleep spindles and K-complexes, on the EEG. The first stage II cycle lasts between ten and twenty-five minutes, and each following cycle becomes longer, until a total of about 45 percent of sleep time is spent in this stage (Patel et al., 2024; Suni, 2023).
Stage III
Brain waves slow considerably during stage III (N3), so this stage is referred to as slow-wave sleep (SWS) (Patel et al., 2024; Suni, 2023). While brain waves slow, metabolic, immune, and restorative actions occur. These include production of growth hormones, regulation of immune function, and repair of muscle tissue (Patel et al., 2024; Lockett, 2023; Suni, 2023). In addition, the depth of sleep during stage III is implicated as contributing to creativity, memory, and higher-level thinking (Suni, 2023). Stage III involves further decrease of temperature, heart and respiratory rates, and relaxation to the point of loss of skeletal muscle tone. This stage of sleep is when the deepest sleep occurs, and it is hard to be awakened, even by loud noise. If awakened during this stage, it is typical for the person to experience sleep inertia, or slowed processing of thoughts (Patel et al., 2024). Urinating while asleep (enuresis) and sleepwalking (somnambulism) happen during stage III sleep (Patel et al., 2024). Individuals can dream in this phase, with dreams typically being realistic in nature. Stage III predominantly occurs within the first half of a night’s sleep, with these early cycles lasting twenty to forty minutes each and getting shorter throughout the night as more REM sleep begins (Suni, 2023). Approximately 25 percent of sleep is spent in stage III.
Stage IV Rapid Eye Movement (REM) Sleep
REM sleep is considered stage IV of the sleep cycle. REM sleep, as the name indicates, is characterized by rapid movements of the eyes. Additionally, during this stage, metabolism increases, as noted by increases and changes in respiratory patterns and cardiac rhythms (Nunez & Lamoreux, 2023). Brain activity also increases during REM. Memory storage, integration of learned concepts, and creativity are associated with REM sleep (Nunez & Lamoreux, 2023; Suni, 2023). A lack of REM sleep can lead to psychophysiological symptoms and diagnoses related to sleep deprivation in general, which will be explored later in the chapter. Because REM sleep is associated with actions such as memory consolidation, particularly those memories related to skill acquisition, and problem-solving (Peters, 2023), a lack of REM sleep can have negative impacts on processing thoughts and emotions, and in making connections. These effects can manifest in a variety of ways, including depression, anxiety, a lack of clarity in thinking, or decreased higher-level cognition (Peters, 2023).
People lose skeletal muscle tone (except the diaphragm and muscles of the eyes), eliminating movement during REM sleep (Brinkman et al., 2023; Lockett, 2023; Patel et al., 2024). Dreams are common and tend to be very clear, emotional, and unbelievable. Nightmares also may occur during this phase, as well as sexual stimulation in the form of penile erections and clitoral dilation (Patel et al., 2024).
Link to Learning
This video shows the rapid movement of the eyes and a summary of typical events associated with REM sleep.
REM sleep increases during the second half of the night, as stage III sleep decreases, increasing from a ten-minute first cycle, to a final REM stage of sixty minutes (Suni, 2023). Approximately 25 percent of sleep is spent in REM, and it is common to awaken in the morning in the midst of REM sleep (Table 28.1).
| Sleep Stage | Type of Sleep | Length of Stage (minutes) | Sleep Time (percent) |
|---|---|---|---|
| Stage I/N1 | NREM | 1–7 | 5 |
| Stage II/N2 | NREM | 10–25 | 45 |
| Stage III/N3 | NREM | 20–40 | 25 |
| Stage IV/REM | REM | 10–60 | 25 |
Link to Learning
For a summary of what is happening during a night’s sleep watch this video. You will need to scroll down about halfway.
Sleep Patterns and Needs
A period of sleep is typically overnight, and sleep occurs in the stages described in cyclical fashion. Such sleep cycles repeat until natural awakening, which frequently correlates with morning light, or until wakened by an alarm or other interruption. In many societies, this monophasic sleep pattern is most common, with one major block of time allocated for sleep.
In some regions of the world, a biphasic sleep pattern may be the norm, with two sleep sessions per day, including a sleep period of several hours overnight, and another, shorter sleep in the later morning or afternoon. Another is the polyphasic sleep pattern, which is normal for infants and young children, who sleep several times in a twenty-four-hour period. Until a neonate’s sleep pattern normalizes to days and nights, even the overnight sleep cycles are not particularly long. Sleep patterns continue to change throughout the life span, with developmental influences influencing amounts and times of sleep from infancy through older adulthood.
Sleep Cycle
A full sleep cycle is comprised of three NREM stages and one REM stage. For most adults, the first cycle encompasses approximately 70 to 100 minutes, and subsequent cycles involve 90 to 100 minutes (Brinkman et al., 2023; Suni, 2023). Adults typically experience four to six cycles of sleep nightly, or during the major session of sleep. The most restorative sleep occurs during the N3 stage, and most of this sleep stage occurs in the first few hours of sleep, with REM increasing in the latter cycles of sleep.
Patient Conversations
What’s Going on While I Sleep?
Scenario: A patient is being interviewed prior to seeing the primary care physician for problems sleeping. The patient has dark circles under the eyes and is yawning frequently.
Nurse: Mr. Richardson, I’m Christy, and I understand you’re having trouble sleeping. Can you tell me more about this?
Patient: Of course. Gosh, it’s been about a month now. It seems every night it takes longer for me to fall asleep. Then, I wake up at the lightest noises. But then, it’s almost funny—my wife says sometimes I’m so hard to wake up!
Nurse: I’m sorry this is happening—it can be so frustrating to not sleep well. Let me tell you a little about what happens when you’re asleep.
Patient: How much can happen? I’m asleep, after all!
Nurse: That’s just it, Mr. Richardson. You’d be surprised how much happens while you’re asleep. So, sleep includes four stages, and they work in a pretty predictable cycle. Normally, it only takes a few minutes to fall asleep—no more than seven to ten minutes.
Patient: It’s taking me a half hour most nights now.
Nurse: We’ll talk about some possible reasons for that shortly, and maybe I can help with some ideas. So, that first stage is N1 sleep, and the next is N2, and during this time your body slows things down. Your eyes stop moving, your body cools down, your heart and breathing slow down. Your brain is working on things that happened during the day, and starts to build memories. This is when you’re waking up easily, though, which can be very bothersome.
Patient: Very! And then I have to fall asleep again.
Nurse: The other thing happening at that time, before you woke up, is your body was getting ready for N3, the next stage, when you sleep really soundly. But it has to start over to get you there, because first you have to fall asleep. N3 is when your wife struggles to wake you up.
Patient: She says I sleep like a rock!
Nurse: During N3 sleep, you’re sleeping very deeply, and your body slows down even more, but your mind is still busy removing toxins and fixing things.
Patient: That’s fascinating. Who knew so much was going on?
Nurse: Your brain is a busy place while you’re sleeping. And in the next stage, your body gets busy again. REM stands for “rapid eye movement.” When you’re in REM sleep, as you can imagine from the name, your eyes are moving quickly, and your brain is busy again, like it is during the day. You breathe faster again, and your heart speeds back up. And this is when your dreams often include actions that don’t make sense, like flying or breathing underwater.
Patient: Sometimes when I wake up, those are the wild dreams I wake up from!
Nurse: It’s quite possible. The dreams you might have during N3, you often don’t remember, and you rarely wake up during them, and you sleep in N3 a lot in the first part of the night. Then you go into REM more for the rest of the night. And, it’s easier to wake up in REM, and since it occurs more as you get close to waking up, that’s often the stage you’re sleeping in when you wake up.
Patient: I had no idea so much was going on overnight, especially in my mind!
Scenario follow-up: The nurse proceeds with further inquiry as to the patient’s sleep history, and offers sleep hygiene ideas that may help with falling asleep. Christy also explains she will see the patient after the physician visit to provide further education and assistance.
Sleep patterns are also referred to as sleep architecture. A sleep study, or polysomnography, involves results of assorted physiologic monitors, including EEG, electrocardiogram (ECG), electrooculogram (EOG), electromyogram (EMG), and oxygen saturation (SaO2). The individual’s sleep architecture is displayed as a graph, called a hypnogram (Figure 28.4).
The first sleep cycle of the night, if undisturbed, includes all three NREM stages, including a repeat of N2, followed by REM, with continued cycles through the stages in this manner: N1—N2—N3—N2—REM (Cherry, 2023). Unless wakened, N1 is not repeated. Healthy transitions between the stages of sleep happen more often if an individual has good sleep hygiene, or healthy sleep habits. Factors affecting sleep hygiene include
- maintaining regular sleep and wake times,
- providing an environment conducive to sleep (e.g., dark, quiet, cool room),
- allowing enough time for adequate sleep,
- reducing substance use (e.g., alcohol, caffeine, and nicotine), and
- addressing sleep-related disorders (e.g., sleep apnea, restless leg syndrome (Suni, 2023).
Developmental Patterns
During growth and development phases, sleep patterns change (Table 28.2). This includes quality of sleep, time preference, sleep stages, and sleep patterns, with resulting adjustments to sleep architecture. With advancing age, from infancy to older adults, the need for sleep tends to lessen to some extent, from the majority of a twenty-four-hour period to about eight hours.
Gender also has some general differences, with men in stage I sleep and waking more times overnight than women. Women tend to take longer entering sleep, and spend longer in SWS. Pregnancy and the postpartum period bring their own challenges, with more sleepiness during the day and struggles to find comfortable positions in advanced stages of pregnancy.
| Infancy (0–12 months) | Neonate |
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| Infant |
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| Childhood (1–9 years) | Toddler (1–3 years) |
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| Child (3–9 years) | ||
| Adolescence | 10–18 years |
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| Adulthood (18+) | Young/middle age adult (18–65) |
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| Older adult (65+) |
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