Sleep Definitions


Referenced sources: CCH (Cincinnati Children's Hospital Glossary); JSD (Jasper GA Sleep Disorders Center Glossary); Sleepydave (ASAA moderator/sleep clinic director)


AHI -- Apnea Hypopnea Index -- The number of apneic episodes (obstructive, central, and mixed) plus hypopneas per hour of sleep, as determined by all-night polysomnography. (JSD); Normal AHI is <5, mild 5-14, moderate 15-30, severe >30.” (Sleepydave)
Apnea -- Cessation of airflow at the nostrils and mouth lasting at least 10 seconds (JSD)
Arousal -- An abrupt change from a ‘deeper’ stage of non-REM (NREM) sleep to a ‘lighter’ stage, or from REM sleep toward wakefulness, with the possibility of awakening as the final outcome. Arousal may be accompanied by increased tonic electromyographic activity and heart rate, as well as by an increased number of body movements. (JSD)
Awakening -- The return to the polysomnographically defined awake state from any NREM or REM sleep stages. (JSD)
Circadian Rhythms -- Circadian rhythms are our bodies natural rhythms that influence a number of biological / physiological processes, such as sleep and wake patterns, body temperature and certain hormonal changes. Circadian rhythms generally follow a cycle that lasts approximately 24 hours. (CCH)
Hypercapnia -- Hypercapnia is the presence of an abnormally high level of carbon dioxide in the circulating blood. (CCH)
Hypopnea -- Hypopnea is partial (30-50%) cessation of airflow at the nose and / or mouth. (CCH); An episode of shallow breathing (airflow reduced by at least 50%) during sleep, lasting 10 seconds or longer, usually associated with a fall in blood oxygen saturation. (JSD)
Hypoxemia -- Hypoxemia is reduced oxygen in the circulating blood. (CCH)
Normal Sleep -- Normal sleep % are: Stage 1 - 5%; Stage 2 - 55%; Stage 3/4 - 20%; REM - 20%. Stage 1 is only a transition state from wake to stage 2 and has no real rest value. A lot of it means inability to initiate and/or maintain sleep. Stage 3/4 (SWS) decreases as a function of age, or if you're subject to constant arousals, as in OSA. There are strict criteria to score SWS, so one may still be having some SWS-like activity, but can't be scored as such because it fails to meet scoring criteria, such as waveform height (amplitude). (Sleepydave)
Oxygen Saturation -- Oxygen Saturation: Oxygen content of blood divided by oxygen capacity and expressed in volume percent. (JSD); Normal Oxygen Saturation (SaO2) is >94%. (Sleepydave)
Sleep Efficiency -- Sleep Efficiency (or Sleep-Efficiency Index): The proportion of sleep in the episode potentially filled by sleep (i.e., the ration of total sleep time to time in bed) or Sleep Efficiency = Total sleep time multiplied by time in bed. (JSD); Normal sleep efficiency is at least 85% (asleep 85% of the night). It is reduced in a number of situations, such as insomnia or simply lab effect. (Sleepydave)
Sleep Latency -- Sleep Latency: The duration of time from ‘lights out,’ or bedtime, to the onset of sleep. (JSD); Normal sleep latency is about 15 minutes, REM latency is 90 minutes, so those are OK. (Sleepydave)
Spontaneous Arousal Index -- Spontaneous Arousal Index: The number of spontaneous arousals (e.g. arousals not related to respiratory events, limb movements, snoring, etc) multiplied by the number of hours of sleep. (JSD); An arousal is a wake or "alpha" pattern for 3 to 15 seconds. You are usually not aware of arousals. As you suggest, there are 3 types of arousals reported out on the sleep studies- those attributed to respiratory events, periodic limb movements and those that are spontaneous. Spontaneous arousals have no directly attributable cause, or cannot be linked to the first 2 reasons for arousals. That said, there may be respiratory events during sleep that generate "spontaneous" arousals. like snores or flow limitations. Indeed, the whole Upper Airway Resistance Syndrome is based on the presence of a large number of spontaneous arousals without the presence of scorable respiratory events such as hypopneas or apneas. Technically, they should be identified using an esophageal ballon to measure negative inspiratory pressure (which just about nobody does) or a nasal or oro-nasal pressure transducer, and look for flow limitations. These are RERAs (Respiratory Effort-Related Arousals). If you take this number, RERA, and add it to the AHI, you have the Respiratory Disturbance Index (RDI). There are a bunch of things that could create the scenario for the appearance of a large number of truly spontaneous arousals, such as medications that deter sleep (pseudoephedrine, caffeine, some antidepressants, too much thyroid medication, etc.), depression and narcolepsy. (Sleepydave)


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