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- DictionarySleep/slēp/
noun
- 1. a condition of body and mind that typically recurs for several hours every night, in which the eyes are closed, the postural muscles relaxed, the activity of the brain altered, and consciousness of the surroundings practically suspended: "I was on the verge of sleep"
- 2. a gummy or gritty secretion found in the corners of the eyes after sleep: "she sat up, rubbing the sleep from her eyes"
verb
- 1. be in a state of sleep; be asleep: "she slept for half an hour"
- 2. provide (a specified number of people) with beds, rooms, or places to stay the night: "studios sleeping two people cost $70 a night"
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Learn the noun and verb meanings of sleep, as well as its synonyms, examples, etymology, and related phrases. See also the kids and medical definitions of sleep.
Oct 1, 2021 · Learn how sleep is a highly active process that involves brain activity and physiological functions, and how it can be influenced by various factors. Explore the characteristics, patterns and disorders of sleep, and how to improve your sleep quality and health.
Sleep. Sleeping Girl, Domenico Fetti, c. 1615. Sleep is a state of reduced mental and physical activity in which consciousness is altered and certain sensory activity is inhibited. During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment.
- Overview
- The nature of sleep
- Developmental patterns of sleep and wakefulness
- Psychophysiological variations in sleep
- NREM sleep
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Sleep is a normal, reversible, recurrent state of reduced responsiveness to external stimulation accompanied by complex and predictable changes in physiology. These changes include coordinated, spontaneous, internally generated brain activity and fluctuations in hormone levels and relaxation of musculature.
How much sleep does a human need?
Studies suggest that healthy adults between ages 26 and 64 need about 7 to 9 hours of sleep per night. Adults over age 65 need roughly 7 to 8 hours. Increasing numbers of people, however, sleep fewer than 7 or more than 8 hours.
In which stage of sleep does a person dream?
Rapid eye movement, or REM, sleep is the stage of sleep during which dreams prevail. Although dreaming can also occur during non-rapid eye movement, or NREM, sleep, the dreaming reports from people waking from REM sleep are more frequent, and the content of their dreams is florid, vivid, and hallucinatory.
sleep, a normal, reversible, recurrent state of reduced responsiveness to external stimulation that is accompanied by complex and predictable changes in physiology. These changes include coordinated, spontaneous, and internally generated brain activity as well as fluctuations in hormone levels and relaxation of musculature. A succinctly defined specific purpose of sleep remains unclear, but that is partly because sleep is a dynamic state that influences all physiology, rather than an individual organ or other isolated physical system. Sleep contrasts with wakefulness, in which state there is an enhanced potential for sensitivity and an efficient responsiveness to external stimuli. The sleep-wakefulness alternation is the most-striking manifestation in higher vertebrates of the more-general phenomenon of periodicity in the activity or responsivity of living tissue.
Sleep usually requires the presence of relaxed skeletal muscles and the absence of the overt goal-directed behaviour of which the waking organism is capable. The characteristic posture associated with sleep in humans and in many but not all other animals is that of horizontal repose. The relaxation of the skeletal muscles in that posture and its implication of a more-passive role toward the environment are symptomatic of sleep. Instances of activities such as sleepwalking raise interesting questions about whether the brain is capable of simultaneously being partly asleep and partly awake. In an extreme form of that principle, marine mammals appear to sleep with half the brain remaining responsive, possibly to maintain activities that allow them to surface for air.
Indicative of the decreased sensitivity of the human sleeper to the external environment are the typical closed eyelids (or the functional blindness associated with sleep while the eyes are open) and the presleep activities that include seeking surroundings characterized by reduced or monotonous levels of sensory stimulation. Three additional criteria—reversibility, recurrence, and spontaneity—distinguish sleep from other states. For example, compared with hibernation or coma, sleep is more easily reversible. Although the occurrence of sleep is not perfectly regular under all conditions, it is at least partially predictable from a knowledge of the duration of prior sleep periods and of the intervals between periods of sleep, and, although the onset of sleep may be facilitated by a variety of environmental or chemical means, sleep states are not thought of as being absolutely dependent upon such manipulations.
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Biology Bonanza
In experimental studies, sleep has also been defined in terms of physiological variables generally associated with recurring periods of inactivity identified behaviorally as sleep. For example, the typical presence of certain electroencephalogram (EEG) patterns (brain patterns of electrical activity) with behavioral sleep has led to the designation of such patterns as “signs” of sleep. Conversely, in the absence of such signs (as, for example, in a hypnotic trance), it is believed that true sleep is absent. Such signs as are now employed, however, are not invariably discriminating of the behavioral states of sleep and wakefulness. Advances in the technology of animal experimentation have made it possible to extend the physiological approach from externally measurable manifestations of sleep such as the EEG to the underlying neural (nerve) mechanisms presumably responsible for such manifestations. In addition, computational modeling of EEG signals may be used to obtain information about the brain activities that generate the signals. Such advances may eventually enable scientists to identify the specific structures that mediate sleep and to determine their functional roles in the sleep process.
In addition to the behavioral and physiological criteria already mentioned, subjective experience (in the case of the self) and verbal reports of such experience (in the case of others) are used at the human level to define sleep. Upon being alerted, one may feel or say, “I was asleep just then,” and such judgments ordinarily are accepted as evidence for identifying a pre-arousal state as sleep. Such subjective evidence, however, can be at variance with both behavioral classifications and sleep electrophysiology, raising interesting questions about how to define the true measure of sleep. Is sleep determined by objective or subjective evidence alone, or is it determined by some combination of the two? And what is the best way to measure such evidence?
How much sleep does a person need? While the physiological bases of the need for sleep remain conjectural, rendering definitive answers to this question impossible despite contemporary knowledge, much evidence has been gathered on how much sleep people do in fact obtain. Perhaps the most-important conclusion to be drawn from the evidence is that there is great variability between individuals and across life spans in the total amount of sleep time.
Studies suggest that healthy adults between ages 26 and 64 need about 7 to 9 hours of sleep per night. Adults over age 65 need roughly 7 to 8 hours. Increasing numbers of people, however, sleep fewer than 7 or more than 8 hours. According to sleep polls taken in the United States in 2009, the average number of persons sleeping less than 6 hours per night increased from 12 percent in 1998 to 20 percent in 2009. During that same period the average number of persons sleeping more than 8 hours decreased from 35 percent to 28 percent. Sleep time also differs between weekdays and weekends. In the United States and other industrialized countries, including the United Kingdom and Australia, adults average less than 7 hours of sleep per night during the workweek. For Americans, that average increases only slightly, by an average of 30 minutes, on weekends. However, sleep norms inevitably vary with sleep criteria. The most precise and reliable figures on sleep time come from studies in sleep laboratories, where EEG criteria are employed.
The amount and characteristics of sleep vary significantly according to age. The newborn infant may spend an average of about 16 hours of each 24-hour period in sleep, although there is wide variability between individual babies. By about the sixth month of life, many infants are able to sustain longer sleep episodes and are beginning to consolidate sleep at night. That sleep period is typically accompanied by morning and afternoon napping. During the first year of life, sleep time drops sharply, and by two years of age it may range from 9 to 12 hours.
Sleep duration recommendations for toddlers (age 1 to 2) range from 11 to 14 hours and for preschoolers (age 3 to 5) from 10 to 13 hours, which includes time spent napping. Only a small percentage of 4- to 5-year-old children nap; for most, sleep is consolidated into a single nighttime period. A gradual shift to a later bedtime begins in school-age children (age 6 to 13), who have been estimated to need between 9 and 11 hours of sleep. Adolescents between ages 14 and 17 need at least 8.5 hours of sleep per night, while young adults (age 18 to 25) need at least 7 hours. Most individuals in those age groups, however, sleep fewer than 7 hours. Sleep durations outside the recommended ranges (e.g., as few as 7 or 8 hours or as many as 12 hours in some school-age children) may be normal. Youths whose sleep deviates far from the normal range (e.g., in school-age children, less than 7 hours or more than 12 hours) may be affected by a health or sleep-related problem.
Similar to adults, children and adolescents in some societies tend to show discrepancies between the amounts of sleep obtained on weekday nights versus the weekend or non-school-day nights, typically characterized by marked increases during the latter. Reduced sleep on weekday nights has been attributed to social schedules and late-night activities, combined with an early school start time. Sleep disorders and modern lifestyle habits (e.g., use of electronic media in the bedroom and caffeinated beverages) also have been implicated in influencing the amount and quality of sleep in those age groups.
In older individuals (age 65 and over), sleep duration recommendations are between 7 and 8 hours. Decreases to approximately 6 hours have been observed among the elderly; however, decreases in sleep time in that population may be attributed to the increased incidence of illness and use of medications rather than natural physiological declines in sleep.
That there are different kinds of sleep has long been recognized. In everyday discourse there is talk of “good” sleep and “poor” sleep, of “light” sleep and “deep” sleep, yet not until the second half of the 20th century did scientists pay much attention to qualitative variations within sleep. Sleep was formerly conceptualized by scientists as a unitary state of passive recuperation. Revolutionary changes have occurred in scientific thinking about sleep, the most important of which has been increased appreciation of the diverse elements of sleep and their potential functional roles.
This revolution may be traced back to the discovery of sleep characterized by rapid eye movements (REM), first reported by American physiologists Eugene Aserinsky and Nathaniel Kleitman in 1953. REM sleep proved to have characteristics quite at variance with the prevailing model of sleep as recuperative deactivation of the central nervous system. Various central and autonomic nervous system measurements seemed to show that the REM stage of sleep is more nearly like activated wakefulness than it is like other sleep. Hence, REM sleep is sometimes referred to as “paradoxical sleep.” Thus, the earlier assumption that sleep is a unitary and passive state has yielded to the viewpoint that there are two different kinds of sleep: a relatively deactivated NREM (non-rapid eye movement) phase and an activated REM phase. However, data, notably from brain-imaging studies, stress that this view is somewhat simplistic and that both phases actually display complex brain activity in different locations of the brain and in different patterns over time.
By the time a child reaches one year of age, NREM sleep can be classified into different sleep stages. NREM is conventionally subdivided into three different stages on the basis of EEG criteria: stage 1, stage 2, and stage 3 (sometimes referred to as NREM 1, NREM 2, and NREM 3, or simply N1, N2, and N3). Stage 3 is referred to as “slow-wave sleep” and traditionally was subdivided into stage 3 and stage 4, though both are now considered stage 3. The distinction between these stages of NREM sleep is made through information gleaned from multiple physiological parameters, including EEG, which are reported in frequency (in hertz [Hz], or cycles per second) and amplitude (in voltage) of the signal.
In the adult, stage 1 is a state of drowsiness, a transition state into sleep. It is observed at sleep onset or after momentary arousals during the night and is defined as a low-voltage mixed-frequency EEG tracing with a considerable representation of theta-wave activity (4–7 Hz). Stage 2 is a relatively low-voltage EEG tracing characterized by typical intermittent short sequences of waves of 11–15 Hz (“sleep spindles”). Some research suggests that stage 2 represents the genuine first stage of sleep and that the appearance of spindles, resulting from specific neural interactions between central (thalamus) and peripheral (cortex) brain structures, more reliably represents the onset of sleep. Stage 2 is also characterized on EEG tracings by the appearance of relatively high-voltage (more than 75-microvolt) low-frequency (0.5–2.0-Hz) biphasic waves. During stage 2 these waves, which are also called K-complexes, are induced by external stimulation (e.g., a sound) or occur spontaneously during sleep. Sleep spindles and spontaneous K-complexes are present in the infant at about six months of age (sometimes earlier). As sleep deepens, slow waves progressively become more abundant. Stage 3 is conventionally defined as the point at which slow waves occupy more than 20 percent of the 30-second window of an EEG tracing. Because of slow-wave predominance, stage 3 is also called slow-wave sleep (SWS). Slow-wave activity peaks in childhood and then decreases with age. Across childhood and adolescence there is progressive movement toward an adult sleep pattern consisting of longer 90-minute sleep cycles, shorter sleep totals, and decreased slow-wave activity.
Distinctions between sleep stages are somewhat arbitrary, and the true physiological boundary between stages is less clear than is described by these criteria. By analogy, the expression “teenager” is often used to refer to someone between ages 13 and 19, but there is only a subtle difference between a child of 12 years and 11 months and a child of 13 years and 0 months. The terminology serves to categorize different features, but it must be recognized that the boundary between categories is less clear physiologically than the distinction in terminology implies.
The EEG patterns of NREM sleep, particularly during stage 3, are those associated in other circumstances with decreased vigilance. Furthermore, after the transition from wakefulness to NREM sleep, most functions of the autonomic nervous system decrease their rate of activity and their moment-to-moment variability. Thus, NREM sleep is the kind of seemingly restful state that appears capable of supporting the recuperative functions assigned to sleep. There are in fact several lines of evidence suggesting such functions for NREM sleep: (1) increases in such sleep, in both humans and laboratory animals, observed after physical exercise; (2) the concentration of such sleep in the early portion of the sleep period (i.e., immediately after wakeful states of activity) in humans; and (3) the relatively high priority that such sleep has among humans in “recovery” sleep following abnormally extended periods of wakefulness.
Sleep is a normal, reversible, recurrent state of reduced responsiveness to external stimulation that is accompanied by complex and predictable changes in physiology. Learn about the behavioral, physiological, and subjective criteria of sleep, as well as the different stages, functions, and disorders of sleep.
Learn the meaning of sleep as a noun and a verb, with synonyms, idioms, and phrasal verbs. See how to use sleep in sentences from the Cambridge English Corpus.
Oct 23, 2020 · REM. Stage 4. REM Sleep. 10-60 minutes. In stage 1, you’ve just dozed off and started transitioning to stage 2, which involves further slowing of activity in the brain and body. It’s much easier to be awoken during these early stages of the sleep cycle. Stage 3 is the deepest part of NREM sleep. In this stage, your muscles and body relax ...
Sleep is the balm that soothes and restores after a long day. It is driven by the body’s internal clock, which takes cues from external elements such as sunlight and temperature. Sleep is critical for memory consolidation, information processing, physical growth, muscle repair, and more. Learn how much sleep you need, how to get better sleep, and what is the purpose of dreams.
