Breathing During Sleep
Of all the physiological differences in humansleep compared with wakefulness that have been discovered in the last decade,changes in respiratory control are most dramatic. Not only are theredifferences in the level of the functioning of respiratory systems, there areeven changes in how they function. Movements of the rib cage for breathing arereduced during sleep, making the contractions of the diaphragm more important.Yet because of the physics of lying down, the stomach applies weight againstthe diaphragm and makes it more difficult for the diaphragm to do its job.However, there are many other changes that affect respiration when asleep.
During wakefulness, breathing is controlledby two interacting systems. The first is an automatic, metabolic system whosecontrol is centered in the brain stem. It subconsciously adjusts breathing rateand depth in order to regulate the levels of carbon dioxide (CO2) and oxygen(O2), and the acid-base ratio in the blood. The second system is the voluntary,behavioral system. Its control center is based in the forebrain, and itregulates breathing for use in speech, singing, sighing, and so on. It iscapable of ignoring or overriding the automatic, metabolic system and producesan irregular pattern of breathing.
During NREM (the phase of sleep in whichthere is no rapid eye movement) breathing becomes deeper and more regular, butthere is also a decrease in the breathing rate, resulting in less air beingexchanged overall. This occurs because during NREM sleep the automatic,metabolic system has exclusive control over breathing and the body uses lessoxygen and produces less carbon dioxide. Also, during sleep the automaticmetabolic system is less responsive to carbon dioxide levels and oxygen levelsin the blood. Two things result from these changes in breathing control thatoccur during sleep. First, there may be a brief cessation or reduction ofbreathing when falling asleep as the sleeper waxes and wanes between sleep andwakefulness and their differing control mechanisms. Second, once sleep is fullyobtained, there is an increase of carbon dioxide and a decrease of oxygen inthe blood that persists during NREM.
But that is not all that changes. Duringall phases of sleep, several changes in the air passages have been observed. Ittakes twice as much effort to breathe during sleep because of greaterresistance to airflow in the airways and changes in the efficiency of themuscles used for breathing. Some of the muscles that help keep the upper airwayopen when breathing tend to become more relaxed during sleep, especially duringREM (the phase of sleep in which there is rapid eye movement). Without thismuscular action, inhaling is like sucking air out of a balloon—the narrow passages tend tocollapse. Also there is a regular cycle of change in resistance between the twosides of the nose. If something blocks the "good" side, such ascongestion from allergies or a cold, then resistance increases dramatically.Coupled with these factors is the loss of the complex interactions among themuscles that can change the route of airflow from nose to mouth.
Other respiratory regulating mechanismsapparently cease functioning during sleep. For example, during wakefulnessthere is an immediate, automatic, adaptive increase in breathing effort wheninhaling is made more difficult (such as breathing through a restrictive facemask). This reflexive adjustment is totally absent during NREM sleep. Onlyafter several inadequate breaths under such conditions, resulting in theconsiderable elevation of carbon dioxide and reduction of oxygen in the blood,is breathing effort adjusted. Finally, the coughing reflex in reaction toirritants in the airway produces not a cough during sleep but a cessation ofbreathing. If the irritation is severe enough, a sleeping person will arouse, clearthe airway, then resume breathing and likely return to sleep.
Additional breathing changes occur duringREM sleep that is even more dramatic than the changes that occur during NREM.The amount of air exchanged is even lower in REM than NREM because, althoughbreathing is more rapid in REM, it is also more irregular, with brief episodesof shallow breathing or absence of breathing. In addition, breathing during REMdepends much more on the action of the diaphragm and much less on rib cageaction.
Paragraphy1: Of all the physiologicaldifferences in human sleep compared with wakefulness that have been discoveredin the last decade, changes in respiratory control are most dramatic. Not onlyare there differences in the level of the functioning of respiratory systems,there are even changes in how they function. Movements of the rib cage forbreathing are reduced during sleep, making the contractions of the diaphragmmore important. Yet because of the physics of lying down, the stomach appliesweight against the diaphragm and makes it more difficult for the diaphragm todo its job. However, there are many other changes that affect respiration whenasleep.
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