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NCBI Bookshelf. Humans spend about one-third of their lives asleep. NREM sleep is divided into stages 1, 2, 3, and 4, representing a continuum of relative depth. Each has unique characteristics including variations in brain wave patterns, eye movements, and muscle tone. Circadian rhythmsthe daily rhythms in physiology and behavior, regulate the sleep-wake cycle.

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In addition, the sleep-wake system is thought to be regulated by the interplay of two major processes, one that promotes sleep and one that maintains wakefulness. Humans spend about one-third of their lives asleep, yet most individuals know little about sleep. Although its function remains to be fully elucidated, sleep is a universal need of all higher life forms including humans, absence of which has serious physiological consequences. Sleep and circadian-generating systems are also reviewed.

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Sleep architecture refers to the basic structural organization of normal sleep. Sleep cycles and stages were uncovered with the use of electroencephalographic EEG recordings that trace the electrical patterns of brain activity Loomis et al.

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Progression of sleep states across a single night in young adult. NREM sleep constitutes about 75 to 80 percent of total time spent in sleep, and REM sleep constitutes the remaining 20 to 25 percent. The second, and later, cycles are longer lasting—approximately 90 to minutes Carskadon and Dement, In normal adults, REM sleep increases as the night progresses and is longest in the last one-third of the sleep episode.

As the sleep episode progresses, stage 2 begins to for the majority of NREM sleep, and stages 3 and 4 may sometimes altogether disappear. The four stages of NREM sleep are each associated with distinct brain activity and physiology. Other instruments are used to track characteristic changes in eye movement and muscle tone. NREM stage 1 sleep serves a transitional role in sleep-stage cycling.

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This stage usually lasts 1 to 7 minutes in the initial cycle, constituting 2 to 5 percent of total sleep, and is easily interrupted by a disruptive noise. Brain activity on the EEG in stage 1 transitions from wakefulness marked by rhythmic alpha waves to low-voltage, mixed-frequency waves. Alpha waves are associated with a wakeful relaxation state and are characterized by a frequency of 8 to 13 cycles per second Carskadon and Dement, Stage 2 sleep lasts approximately 10 to 25 minutes in the initial cycle and lengthens with each successive cycle, eventually constituting between 45 to 55 percent of the total sleep episode.

An individual in stage 2 sleep requires more intense stimuli than in stage 1 to awaken. Brain activity on an EEG shows relatively low-voltage, mixed-frequency activity characterized by the presence of sleep spindles and K-complexes Figure It is hypothesized that sleep spindles are important for memory consolidation. Individuals who learn a new task have a ificantly higher density of sleep spindles than those in a control group Gais et al.

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Sleep stages 3 and 4 are collectively referred to as slow-wave sleep SWSmost of which occurs during the first third of the night. Each has distinguishing characteristics. Stage 3 lasts only a few minutes and constitutes about 3 to 8 percent of sleep. The EEG shows increased high-voltage, slow-wave activity Figure The last NREM stage is stage 4, which lasts approximately 20 to 40 minutes in the first cycle and makes up about 10 to 15 percent of sleep.

The arousal threshold is highest for all NREM stages in stage 4. This stage is characterized by increased amounts of high-voltage, slow-wave activity on the EEG Carskadon and Dement, REM sleep is defined by the presence of desynchronized low-voltage, mixed-frequency brain wave activity, muscle atonia, and bursts of rapid eye movements Carskadon and Dement, During the initial cycle, the REM period may last only 1 to 5 minutes; however, it becomes progressively prolonged as the sleep episode progresses Carskadon and Dement, Dreaming is most often associated with REM sleep. Approximately 80 percent of vivid dream recall after arousal from this stage of sleep Dement and Kleitman, b.

In addition to the physiological changes listed in Tablethere are other body system changes that occur during sleep. Generally, these changes are well tolerated in healthy individuals, but they may compromise the sometimes fragile balance of individuals with vulnerable systems, such as those with cardiovascular diseases Parker and Dunbar, Physiological changes also occur in the following systems:.

The sleep-wake system is thought to be regulated by the interplay of two major processes, one that promotes sleep process S and one that maintains wakefulness process C Gillette and Abbott, Process S is the homeostatic drive for sleep. The need for sleep process S accumulates across the day, peaks just before bedtime at night and dissipates throughout the night.

Process C is wake promoting and is regulated by the circadian system. Process C builds across the day, serving to counteract process S and promote wakefulness and alertness. However, this wake-promoting system begins to decline at bedtime, serving to enhance sleep consolidation as the need for sleep dissipates across the night Gillette and Abbott, In the absence of process C, total sleep time remains the same, but it is randomly distributed over the day and night; therefore, process C also works to consolidate sleep and wake into fairly distinct episodes Gillette and Abbott, Importantly, through synchronization of the circadian system, process C assists in keeping sleep-wakefulness cycles coordinated with environmental light-dark cycles.

Sleep process S is regulated by neurons that shut down the arousal systems, thus allowing the brain to fall asleep. Many of these neurons are found in the preoptic area of the hypothalamus Figure A. These neurons, containing molecules that inhibit neuronal communication, turn off the arousal systems during sleep. Loss of these nerve cells causes profound insomnia Saper et al.

Inputs from other regions of the brain also greatly influence the sleep system. These include inputs from the lower brainstem that relay information about the state of the body e. In addition, as described further in the next section, there are inputs from the circadian system that allow the wake-sleep system to synchronize with the external day-night cycle, but also to override this cycle when it is necessitated by environmental needs.

Sleep-generating A and wake-generating B systems in the brain. The sleep-generating system also includes neurons in the pons that intermittently switch from Ladies looking sex Buxton to REM sleep over the course of the night. These neurons send outputs to the lower brainstem and spinal cord that cause muscle atonia, REMs, and chaotic autonomic activity that characterize REM sleep.

Other outputs are sent to the forebrain, including activation of the cholinergic pathways to the thalamus to activate the EEG. Wakefulness is generated by an ascending arousal system from the brainstem that activates forebrain structures to maintain wakefulness Figure B. This idea, originally put forward by Morruzzi and Magounhas more recently been refined Jones, a ; Saper et al. The main source for the ascending arousal influence includes two major pathways that originate in the upper brainstem. The first pathway, which takes origin from cholinergic neurons in the upper pons, activates parts of the thalamus that are responsible for maintaining transmission of sensory information to the cerebral cortex.

The second pathway, which originates in cell groups in the upper brainstem that contain the monoamine neurotransmitters norepinephrine, serotonin, dopamine, and histamineenters the hypothalamus, rather than the thalamus, where it picks up inputs from nerve cells that contain peptides orexin or hypocretin and melanin-concentrating hormone. These inputs then traverse the basal forebrain, where they pick up additional inputs from cells containing acetylcholine and gamma-aminobutyric acid.

Ultimately, all of these inputs enter the cerebral cortex, where they diffusely activate the nerve cells and prepare them for the interpretation and analysis of incoming sensory information. Circadian rhythms refer, collectively, to the daily rhythms in physiology and behavior.

They control the sleep-wake cycle, modulate physical activity and Ladies looking sex Buxton consumption, and over the course of the day regulate body temperature, heart rate, muscle tone, and hormone secretion. The rhythms are generated by neural structures in the hypothalamus that function as a biological clock Dunlap et al. Animals and plants possess endogenous clocks to organize daily behavioral and physiological rhythms in accord with the external day-night cycle Bunning, In mammals, two proteins, Clock and Bmal1, bind together and move into the nucleus of the cell, where they bind to specific sites in the DNA that activate specific genes Figure Among the genes that they activate are Period and Cryptochrome.

The products of these genes also move back into the nucleus, where they disrupt the binding of Clock and Bmal1 to the DNA, thus inhibiting their own synthesis. This in a rising and falling pattern of expression of the Period and Cryptochrome gene products with a periodicity Ladies looking sex Buxton is very close to 24 hours.

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Molecular mechanisms underlying the activity of the circadian clock. Many other genes are also regulated by Clock and Bmal1, and these genes cycle in this way in many tissues in the body, giving rise to daily patterns of activity. These rhythmically expressed genes contribute to many aspects of cellular function, including glucose and lipid metabolism, al transduction, secretion, oxidative metabolism, and many others, suggesting the importance of the circadian system in many central aspects of life.

The suprachiasmatic nucleus SCN is responsible for regulating circadian rhythms in all organs. It receives direct inputs from a class of nerve cells in the retina that act as brightness detectors, which can reset the clock genes in the SCN on a daily basis. The SCN then transmits to the rest of the brain and body als that bring all of the daily cycles in synchrony with the external day-night cycle. The main influence of the SCN on sleep is due to a series of relays through the dorsomedial nucleus of the hypothalamus, which als to the wake-sleep systems to coordinate their activity with the day-night cycles.

The SCN also coordinates cycles of feeding, locomotor activity, and hormones, such as corticosteroids Chou et al. Under some conditions e. In such circumstances, the dorsomedial nucleus may shift to a new daily cycle, which can be completely out of phase with the SCN and the light-dark cycle, and its als also shift the daily cycles of sleep, activity, feeding, and corticosteroid hormone secretion Saper et al.

Another major output of the SCN is to a pathway that controls the secretion of melatonin, a hormone produced by the pineal gland. Melatoninwhich is mainly secreted at night, acts to further consolidate the circadian rhythms but has only limited effects directly on sleep. Body temperature regulation Ladies looking sex Buxton subject to circadian system influence.

At night there is a gradual decline in body temperature, a decrease in heat production called the falling phase of the body temperature rhythmand an increase in heat loss, all which promote sleep onset and maintenance, as well as EEG slow-wave activity. Conversely, there is a gradual increase in body temperature several hours before waking. The brain sends als to other parts of the body Ladies looking sex Buxton increase heat production and conservation in order to disrupt sleep and promote waking Szymusiak, Body temperature in relation to time of day.

Sleep architecture changes continuously and considerably with age. From infancy to adulthood, there are marked changes in how sleep is initiated and maintained, the percentage of time spent in each stage of sleep, and overall sleep efficiency i. A general trend is that sleep efficiency declines with age Figure Although the consequences of decreased sleep efficiency are relatively well documented, the reasons are complex and poorly understood. Exami nation of sleep characteristics by age, however, allows a closer understanding of the function of sleep for human development and successful aging.

Changes in sleep with age. At birth, sleep timing is distributed evenly across day and night for the first few weeks, with no regular rhythm or concentration of sleeping and waking. Newborns sleep about 16 to 18 hours per day; however, it is discontinuous with the longest continuous sleep episode lasting only 2.

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This distinctive sleep architecture occurs mostly because circadian rhythms have not yet been fully entrained Davis et al. Circadian rhythms begin to arise around 2 to 3 months of age, leading to sleep consolidation that manifests in greater durations of wakefulness during the day and longer periods of sleep at night Sheldon, Circadian rhythm development in the first 3 months includes: emergence of the hour core body temperature cycle 1 month of age ; progression of nocturnal sleeping 2 Ladies looking sex Buxton of age ; and cycling of melatonin and cortisol hormones in a circadian rhythm 3 months of age Jenni and Carskadon, Sleep cycles also change because of the emergence of the circadian rhythm and a greater responsiveness to social cues such as breast-feeding and bedtime routines.

By 6 months of age, total sleep time reduces slightly and the longest continuous sleep episode lengthens to approximately 6 hours Anders et al. By 12 months old, the infant typically sleeps 14 to 15 hours per day with the majority of sleep consolidated in the evening and during one to two naps during the day Anders et al. There are a limited of studies that address normal sleep architecture in young children; however, one trend that appears to be consistent is that sleep amounts decrease as gets older.

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