Sleep may be defined as a state of natural unconsciousness from which a person can be aroused. Despite the advances that have been made in related areas, sleep remains a complicated physiological entity that is not yet fully understood.
For many years sleep was thought to be a purely passive state. Now, however, sleep is known to be an active process and that the brain is actually quite busy during sleep. Sleep is now known to affect both physical and mental health, and is essential for the normal functioning of all the systems of the body. Since the body is known to require sleep as part of its homeostatic regulatory and repair mechanisms, it seems logical that the body can exert considerable influence on the sleep process.
The Physiology of Sleep
Sleep is a part of the daily routine in everyone, even when the “normal” sleep/wake pattern is disrupted by outside factors. In this section we will use the “normal” circadian rhythm (from the Latin words circa “about” and dies “the day”) of day/night activities to explore the chemistry and physiology of sleep. In humans, the circadian cycle operates in a 24 to 25 hour cycle measured from awakening after one sleep period to awakening from the next sleep period.
The physiologic mechanisms of circadian rhythm begin when light strikes special cells within the retina of the eye which, in turn, causes these cells to secrete a special hormone that causes an area of the brain known as the suprachiasmatic nucleus to signal the pineal body to stop secreting the hormone melatonin, which has been shown to reach its highest levels during sleep. As the day progresses, a chemical called adenosine accumulates within the brain as the level of melatonin falls.
As night falls the inhibitory effects of the retinal secretions on the pineal body are removed allows the pineal to begin secreting melatonin once again. It is the presence of higher melatonin levels within certain areas of the brain (such as the thalamus and hypothalamus) that controls the urge to sleep.
The Stages of Sleep
Many textbooks of sleep medicine refer to the “Architecture of Sleep.” In this “model” sleep is divided into two periods: Non-REM sleep and Rapid Eye Movement (REMS) sleep. Non-REM Sleep (NREMS) is further subdivided into 4 stages, each with its own distinguishing characteristics.
Stage 1 is sometimes called the “gateway to sleep” stage because 1) it appears at the onset of the sleep cycle and 2) it serves as a transitional stage between wakefulness and sleep. A more recent term for this stage has been adopted by many sleep researchers: hypnagogic sleep. Hypnagogic sleep is the stage where some people will experience a phenomenon called hypnic jerks. These “jerks” are a sudden contraction of the muscles often preceded by a sensation of falling. Such spasmodic movements are harmless and do not indicate the presence of other conditions. During this stage the body begins to lose its muscle tone and conscious awareness of external events.
Stage 2 sleep is the longest stage of sleep and is characterized by a further decrease in the body’s muscle tone and awareness of the external environment.
Stage 3 is best thought of as a transitional phase between Stage 2 and Stage 4 sleep.
Stage 4 is the deepest stage of NREMS and is the stage of deepest disassociation of consciousness. This is also the stage that it is most difficult to waken a sleeper.
REM sleep (REMS) is the stage where the majority of dreaming occurs, although dreams may occur during any stage of sleep. While REMS is considered to be the “final” or “deepest” stage of sleep, there are several important changes in the body’s internal environment that occur only at during this stage. These include a depression of voluntary muscle tone that is irregularly interrupted by muscular twitching, increased activity in the cerebral cortex (probably associated with dreaming), and of course rapid eye movements.
Sleep Requirements and Age
It is well known that sleep requirements demonstrate considerable variability and that much of this variability is age-related. In other words, sleep requirements seem to greatest in newborns and decrease with age. In newborns and many infants (to about the age of 1 year) the sleep cycle is “fragmented” into several periods of alternating sleep/wakefulness. Adults generally tolerate sleep periods of 6-8 hours quite well while older adults generally require ~ 6 hours of sleep per 24 hours although they tend to experience a few brief interruptions over the course of a sleep cycle.
The Function of Sleep
There have been several theories advanced concerning the biological function of sleep. While no one theory can explain all the observations that have made regarding sleep, some are more widely accepted than others.
The Restorative Theories of sleep hold that sleep is a time of growth and repair. Some have cited the rise in certain growth hormones that have been shown to increase during periods of deeper sleep as supporting these hypotheses. However, no one has been able to demonstrate any significant degradation of organ function during periods of sleep deprivation. These theories currently have very little support in the scientific community.
The Preservation Theory states that our sleep cycles (and the desire to sleep at night or under conditions resembling night) are the result of an evolutionary process that grew from our remote ancestor’s habits of resting/sleeping at night, a time when predator species enjoyed an advantage in vision and stealth. Over time, the “sleep when it’s dark” and “work in the daylight” behaviors were amplified by natural selection and are present in the brain’s neurochemistry.
The Memory Encoding explanation draws upon the large body of evidence demonstrating that learning is facilitated when the body is well-rested and that memory retention is enhanced by resting after some new lesson is learned.
Of course, none of the above theories adequately explains all the facets of sleep as a phenomenon. It is also just as likely that a single theory will never be proposed that will incorporate the ever-expanding knowledge base related to the physiology and function of sleep as a biologic function.
A sleep disorder is loosely defined as any condition or process that alters the patient’s previously established sleep/wake cycle. As with many other medical conditions, sleep disorders span a spectrum that ranges from inconvenience to potentially life threatening. Sleep disorders are divided into two general classes: dysomnias and parasomnias.
Dysomnias are conditions that manifest themselves as either hypersomnia (abnormal sleep cycles causing the urge to sleep at times when the circadian cycle would suggest that wakefulness was appropriate) or insomnia (the inability to sleep). The dysomnias can be further subdivided into three classes that are dependent on the source of the sleep interference:
Intrinsic (arising within the body): primary insomnia, obstructive sleep apnea, restless leg disorder, and unspecified limb movements
Extrinsic (arising outside the body): environmental conditions not conducive to uninterrupted sleep such as noise of ambient temperature.
Alteration or interference with the circadian rhythm: jet lag or variations in occupational schedules (shift work).
Parasomnias include sleep terror (sudden awakening and unreasonable fear), bedwetting, somnambulism (sleep walking), and somniloquy (talking in one’s sleep).
In summation, sleep and sleep disorders are better understood now than in any other time in human history. Although considerable progress is made daily in sleep research, many manifestations of sleep pathology are still poorly understood.
Bibliography
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Colten, HR and Altevgot, BM (Eds): Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. Washington DC: National Academies Press (2006).
Gillette, M. and Abbot, S. Fundamentals of the circadian system. In: Sleep Research Society (eds.) SRS Basics of SleepGuide. Westchester, IL: Sleep Research Society. (2005).
Myers, DG. Psychology (7th Edition). New York: Worth Publishers (2004).
Society for Neuroscience. Brain Facts: A Primer on the Brain and Nervous System. Washington, DC (2005).
Disclaimer
The information presented in this article and its included links is of an informational nature only and is not intended as a recommendation of any changes in the reader’s health care program. Before making any changes in diet, medications, or other treatments the reader is strongly advised to consult with their health care provider.
