Sleep Related Breathing Disorders Vivek Nangia, Shivani Swami
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Normal Human SleepCHAPTER 1

Vikram Sarbhai,
Naveen P Shah,
Poulomi Chatterjee
 
NORMAL HUMAN SLEEP
Normal human sleep can probably be described as “a reversible behavioral state of perceptual disengagement and unresponsiveness to the environment”. It is also true that sleep is a complex amalgam of physiologic and behavioral processes. Typically, sleep is usually accompanied by postural recumbence, involuntary behavioral quiescence usually get closed eyes, along with physiological alterations one commonly associates with sleeping.
It is certain that sleep is essential, not only for humans but for almost all animals. Physiologically, sleep is a complex neural process that is essential for the restoration and renewal of body functions, which seems to be necessary for life. Scientists still do not have a definitive explanation for why there seems to be an essentiality of the phenomenon of sleep in almost all living forms besides humans as well. We do know that sleep is not a passive process or “switching off” of body functions. In fact, sleep is believed to be important for many physiologic functions including the processing of experiences and the consolidation of diverse neural processes and its manifestations.
Important things to know about normal human sleep:
  • Sleep duration and characteristics decrease with age—sleep characteristics stabilizes until around 20 years of age
  • Sleep differentiation is highly individualized
  • It is normal for children to have daytime naps until 3–5 years old
  • If a child takes naps often past this age, he or she might not be sleeping enough at night
  • Teenagers will tend to go to bed later
  • Older people spend more time in bed, but their sleep requirement is normally similar to that of early adult life.
 
Age Related Sleep Alterations
It is well known that, as children get older their sleep duration is reduced. Different people have different sleep duration requirements. The average sleep durations in different age groups according to Sleep Health Foundation (2011) is described in table 1. Daytime functioning and actions of people can be an indicator to assume their adequacy of sleep at night.
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Table 1   Average sleep requirement (Sleep Health Foundation, 2011)
Age group
Total sleep (hours/day)
Sleep at night (hours)
Sleep during day (hours)
Newborns (0–2 months)
12–18
6–9
6–9
Infants (2–12 months)
14–15
9–12
2.5–5
Toddlers (1–3 years)
12–15
9.5–11.5
1.5–3.5
Preschool (3–5 years)
11–13
Most sleep is at night
Daytime naps become rarer. A child tends to stop napping at this age
School-age (5–12 years)
9–11
All sleep should be at night
Naps at this age tend to be from not getting enough sleep at night
Teenagers (12–18 years)
8.5–9.5
All sleep should be at night
Naps at this age tend to be from not getting enough sleep at night
Adults
7–9
All sleep should be at night
Naps at this age tend to be from not getting enough sleep at night
Note that these are average sleep requirements: some people require more and others less.
 
Age Related Sleep Changes
From birth to two months of age, one period of sleep can vary from 30 minutes to 3–4 hours both during day as well as night. Bottle-fed babies tend to sleep longer than breast-fed babies (3–4 hours vs. 2–3 hours). Two months onwards babies start to sleep longer at a time. This is especially so at night between 12 midnight and 5 AM. This is possibly because babies start to develop their internal circadian (day-night) rhythm around this time. This circadian rhythm favors sleep at night and being more awake during the day. By 6 months, babies can get 5–8 hours of sleep at night. However, in around 25–50% cases, 6 month old children still wake up at night. There are behavioral strategies that can be done to improve and increase night-time sleep, including training babies learn to go to sleep in their cot by themselves. With these measures, they are able to self-soothe themselves back to sleep after waking up during the night. From 2 months to 12 months, the number of daytime naps gets reduced from 3–4 naps to 2 naps. Usually by 12–18 months of age, morning naps stop. It is preferred to allow an afternoon nap after lunch and before 4 pm. Daytime naps become less common from about 2 or 3 years onwards. Persistent daytime naps beyond 5 years of age are not considered normal. It usually means, the child might not be getting enough sleep at night and is suffering from sleep deprivation, quantitative or qualitative. This may be due to poor sleep routines, sleep problems, or sleep disorders.
 
Adult Sleep
Sleep requirements tends to stabilize by 20 years of age. Most adults require between 7 hours and 9 hours a night to feel properly refreshed and function optimally the next day, however, individuals may vary in their sleep needs. 3Some people are genuine short sleepers while others may require considerably more than the average sleep time. Several people try to get away with less sleep.
The reasons for this individual variability in sleep requirement are not well understood. Sleep requirements get reduced with age and older adults spend more time in bed but tend to sleep less. There is a constant reduction in share of rapid eye movement (REM) sleep as compared to non-rapid eye movement (NREM) sleep with age.
 
STAGES OF SLEEP
Sleep time is classified into stages (Figure 1). For scientific applications, sleep is typically scored in recording of 30 seconds (termed as Epoch) with stages of sleep defined by the visual scoring of three parameters: (i) electroencephalogram (EEG); (ii) electrooculogram (EOG); and (iii) electromyogram (EMG) recorded beneath the chin. Sleep can be classified as NREM sleep and REM sleep. During wakefulness, the EEG shows a low voltage fast activity or activated pattern. Voluntary eye movements and eye blinks are obvious. The EMG has a high tonic activity with additional phasic activity sleep related to voluntary movements. As the eyes are closed in sleep preparation, alpha waves [8–13 cycles per second (cps)] become prominent, particularly in occipital regions. NREM sleep, which usually precedes REM sleep, is divided into three stages [namely stage 1, 2, and slow wave sleep (SWS)]. Sleep is usually entered through a transitional state, stage 1 sleep. Stage 1 sleep is characterized by loss of alpha activity and the appearance of a low voltage mixed frequency EEG pattern with prominent theta activity (3–7 cps) and occasional vertex sharp waves. Eye movements become slow and rolling, and skeletal muscle tone relaxes. Subjectively, stage 1 may not be perceived as sleep, however, there is a reduced sensory stimuli, especially visual and mental activity is more dream-like. There may be persistence of motor activity for a number of seconds during stage 1 NREM sleep.
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Figure 1: Various stages of sleep.
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Figure 2: Sleep spindle (12–14 cps) and K-complex (high amplitude negative sharp wave followed by positive slow waves).
Sometimes, individuals may experience sleep jerks, which are sudden muscle contractions, accompanied by a sense of falling and/or dream-like imagery. Such hypnic (hypnosis = mental state like sleep) jerks are generally benign and may be exaggerated by sleep deprivation. After a few minutes of stage 1 NREM, sleep usually progresses to stage 2 NREM, which is characterized by the appearance of sleep spindles (12–14 cps) and K-complexes (high amplitude negative sharp waves followed by positive slow waves) in the EEG (Figure 2). NREM sleep stages 2, SWS, and REM sleep are all subjectively perceived as sleep. Generally, stage 2 NREM is followed by SWS, predominantly more in the first one-third of the night. Delta waves (<2 cps in humans) are characteristic of SWS. In the earlier classification, SWS was subdivided into NREM stage 3 and 4. SWS is also known as deep sleep. The threshold (i.e., sleep intensifies) of arousals increases incrementally from stage 1 through SWS. Eye movements cease during stages 2 and SWS, with further decrease in EMG (muscle) activity. REM sleep is not subdivided into stages, but is rather described in terms of Tonic REM (persistent) and Phasic REM (episodic) components. Tonic aspects of REM sleep include the activated EEG similar to that of stage 1, which may exhibit increased activity in the theta band (3–7 cps), and a generalized atonia of skeletal muscles except for the extra-ocular muscles and the diaphragm. Phasic REM is seen as irregular bursts of rapid eye movements and muscle twitches.
 
Physiological Changes Associated with Sleep
During NREM sleep and tonic REM sleep, there is a relative increase in parasympathetic activity. In comparison to wakefulness, autonomic nervous system reaches its most stable state during SWS. The autonomic system gets unstable (autonomic instability) during phasic REM sleep, where there are brief surges in both sympathetic and parasympathetic activity.
Physiologically, blood pressure, heart rate, and cardiac output decrease during NREM sleep, reaching their lowest average values and least variability in SWS. Although these parameters remain reduced on average, during REM sleep in comparison to waking, they attain their peak values during 5REM. Irregularity in heart rhythm and arrhythmias are more prevalent in REM sleep.
Physiologically, onset of sleep may show temporary breathing instability and/or periodic breathing. The respiratory rate and minute ventilation also decrease during sleep. Muscle relaxation during sleep results in increase in upper airway resistance, most significantly during REM sleep.
These changes contribute to exacerbation of underlying pulmonary disease as well as sleep related breathing disorders due to sleep related upper airway instability such as obstructive sleep apnea.
During NREM sleep, brain and body temperature are downregulated, particularly in NREM SWS, as a result of a decreased hypothalamic temperature set point. There is additional active heat loss through perspiration because of increased cutaneous blood flow (vasodilation). It is usual to experience this phenomenon when people go to sleep feeling somewhat cold and later wish to remove their extra covers as they feel too warm on waking up several hours later. During REM sleep, there is a decreased ability to regulate body temperature through sweating and shivering.
 
Sleep Influence on Endocrine System
Most hormones also show significant interactions with sleep-wakefulness patterns.
Growth hormone (GH) is released during the early part of the sleep and its secretion gets enhanced by NREM SWS. Sleep also stimulates prolactin secretion. Prolactin usually peaks after GH, usually during the middle portion of the sleep cycle. Pulses of GH and prolactin releases can occur after the onset of sleep, regardless of its timing. Both GH and prolactin may have feedback effects on sleep as well; GH seems to enhance SWS, whereas prolactin may increase REM sleep. Contrastingly, thyroid stimulating hormone (TSH) reaches its peak level in the evening just prior to sleep onset; its secretion is inhibited by sleep and gets stimulated by sleep deprivation. On sleep onset, the hypothalamic-pituitary-adrenal axis (HPA axis) is usually at its most inactive state. Sleep onset inhibits cortisol release, however, adrenocorticotrophic hormone (ACTH) and cortisol levels rise at the end of the sleep period, just before awakening and are presumed to contribute to morning awakening. Severe sleep fragmentation or sleep deprivation may have significant clinical consequences on the endocrine system. GH and prolactin levels are decreased in patients with obstructive sleep apnea. Sleep deprivation produces evidence of HPA axis activation in the evening of the day following deprivation. Phenomenon of penile reactions, right from infancy till old age, is one of the characteristics of REM sleep in men. Nocturnal penile tumescence studies are, therefore, helpful in determining whether cases of impotence are related to organic or psychogenic reasons. Similarly, in women, REM sleep produces increased vaginal blood flow and clitoral erection. These changes are not necessarily linked to sexual content in associated dreams.
 
How Sleep Problems are Diagnosed?
Several different medical tests are there to evaluate sleep and assess whether a sleep disorder is present. Most important is a carefully obtained medical history 6and physical examination to identify a medical conditions that may be interfering with the person's sleep. History of use of prescription and non-prescription medications as well as alcohol, tobacco, and caffeine use is important. Laboratory tests may also be used to help diagnose any medical conditions that may cause sleep problems.
Specialized testing of sleep is termed as sleep study, which are performed to identify a sleep disorder. A sleep study is called polysomnography (PSG). The most common sleep tests include the following:
  • Polysomnography: In this test/study, electrodes are attached to the face, eyes, and scalp to measure brain waves such as EEG, EOG, and muscle tone (EMG) during a night's sleep. Other body functions parameters such as airflow, breathing effort, blood oxygen levels, leg movements, electrocardiogram (ECG), and body position are also be measured. Sleep studies are most commonly conducted in specially designed labs in hospitals or sleep clinics
  • The multiple sleep latency test (MSLT) are a series of sleep studies (PSG) designed to measure daytime sleepiness. The test is based upon the fact that the sleepier an individual is, the faster he or she will fall asleep. During this test, the patient is given 4–5 opportunities to nap in a quiet, dark room, usually at 2 hour intervals during the daytime. Body functions such as EEG, EOG, and muscle tone are studied as in PSG. The time period of sleep onset from wakefulness is measured to determine the “sleep latency”. Similar PSG are repeated during each of the 4–5 naps, and an average time for sleep latency across all the naps is calculated. Usually, sleep latency of 5 minutes or less is marked as significant to suggest severe daytime sleepiness
  • Maintenance of wakefulness test (MWT) is similar to the MSLT and is conducted to assess the individual's ability to stay awake when reclining in a quiet, darkened room
  • The Epworth sleepiness scale is the most popular questionnaire that is given to patients, as part of clinic visit by the patient. The test comprises of several questions to rate how likely they would be to fall asleep in a number of diverse situations (such as travel in a car, sitting quietly after lunch, etc.).
 
SLEEP HYGIENE
Nonmedical lifestyle behavioral practices that are believed to help in sleep are referred to as sleep hygiene. These lifestyle adjustments offer the maximum potential for restorative and sound sleep. Good sleep hygiene practices include:
  • Avoid caffeine, nicotine, and alcohol before bedtime. Caffeine consumed early in the day can also have an effect on the ability to fall asleep at night
  • Adhere to a regular bedtime and waking schedule
  • Maintain a comfortable sleep environment, including a comfortable temperature of the room
  • Do not lie in bed awake or worry about not sleeping (or anything else negative) while in bed. This produces anxiety and can make the problem of sleeplessness worse
  • Get regular daily exercise in the morning, however, avoid exercise 2 hours prior to bedtime.
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SLEEP MYTHS
Myth 1: Sleep is a time when our body and brain stops working for rest and relaxation. Evidence, in fact, shows some physiological processes actually become more active while one sleeps proving the belief of organ shut down wrong. For example, secretion of certain hormones is boosted, the pathways in the brain linked to learning and memory shows an increased activity.
Myth 2: Sleeping one hour less won't affect your normal daytime functioning. One hour of less sleep may not make one sleepy, but one's ability to think and decide quickly may be affected, if lack of sleep continues, cardiovascular health and energy balance as well as one's ability to fight infections declines. If one consistently does not get enough sleep, a sleep debt builds up that one can never repay. This sleep debt affects one's health and quality of life and makes one feel tired during the day.
Myth 3: One's body adjusts quickly to different sleep schedules. Normally, one's biological clock makes one most alert during the daytime and least alert at night. Thus, even if one works in night shift he will feel sleepy during night. Most people can reset their biological clock, but by 1–2 hours per day at maximum. Thus, at least more than a week time is required for one to adjust to a substantial change in ones sleep–wake cycle—for example, when traveling across several time zones or switching from working the day shift to the night shift.
Myth 4: People need less sleep as they get older. Physiologically, as people age, the quality of their sleep changes. They find their sleep less refreshing and insomnia or medical conditions disrupting their sleep which is quite common, but they don't need less sleep.
Myth 5: Extra sleep for one night can cure one with excessive daytime tiredness. The quality of sleep is as important as the quantity of sleep. This explains why some people may sleep 8 or 9 hours a night but don't feel well rested as their quality of sleep is poor. The quality of sleep is affected by a number of sleep disorders and some medical conditions, unless these conditions are corrected, excessive daytime fatigue won't be lessened. Adequate behavioral changes or medical therapy can treat most of these conditions.
Myth 6: By sleeping more on the weekends one can make up for lost sleep during the week. Inadequate sleep of many days can't be met by sleeping more on weekends even though one may feel more rested. On the other hand it further complicates ones biological clock and it is much harder to go to sleep at the right time on Sunday nights and get up early on Monday mornings.
Myth 7: Naps wastes ones time. Although naps can never substitute a good night's sleep, but can be refreshing and can be restorative. But one should avoid taking naps later than 3 PM, especially if one has difficulty falling asleep at night, as late naps can make it harder for you to fall asleep when you go to bed. Naps longer than 20 minutes should be avoided as it is more difficult to get into the swing of things after a long nap. A hidden sleep disorder should be suspected if one takes too many naps during the day.
Myth 8: Snoring implies sound sleep. As one gets older snoring during sleep is common. It is established that snoring on a regular basis can increase the risk for diabetes and heart disease and make one tired during the day. Child snorers have been established as poor achievers in school in studies. In fact loud snoring may signify obstructive sleep apnea which is a serious sleep disorders and warrants treatment.
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Myth 9: Children who don't sleep enough at night are sleepy during the day. In contrast to adults, children not getting enough sleep at night are not sleepy but become hyperactive, irritable, and inattentive during the day. They also have increased risk of behavioral problems, are prone to injuries, and may have impaired growth rate. Sleep debt appears to be quite common during young age and at times misdiagnosed as attention-deficit hyperactivity disorder.
Myth 10: Insomnia is most commonly caused by worry. True, stress and worry may be a cause for temporary insomnia, but a persistent insomnia should be prodded more deeply. Certain medications and sleep disorders are known to cause insomnia. Depression, anxiety disorders and asthma, arthritis, or other medical conditions are known to cause insomnia.
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