Understanding Your Sleep Study Results: RDI, Desaturation, and Sleep Architecture - Page 2
© Kerrin Leon White
However, a person might have quite frequent apneas, or especially hypopneas, without much desaturation, especially if none of these respiratory events are prolonged. These briefer events still disrupt the continuity of sleep, causing daytime sleepiness, and can trigger physiological changes like rises in blood pressure, which is one mechanism by which sleep apnea contributes to hypertension.
How to Interpret Sleep Architecture
The interpretation of sleep architecture--the proportionate amounts and timing of the different sleep stages (I, II, III, IV, and REM) throughout the night--presents much more complexity than that of respiratory events. However, you can start by realizing that a normal person has a certain proportion of each stage during the course of a night's sleep, whereas a person with a sleep disorder may have very little or none of one or another stage, especially Rapid Eye Movement (REM), or dreaming sleep, and the deeper stages (III and IV) of non-REM, so-called "slow wave" sleep. These are among the most common abnormalities seen in cases of sleep apnea.
Normally, a person enters sleep through stage 1, enters the first episode of REM 90 minutes later, and throughout the night alternates REM and NREM sleep at intervals of 90 minutes. However, as the night wears on, REM episodes grow progressively longer, so that slow wave sleep predominates in the first third of the night and REM in the last third.
What untreated sleep disorders often do to sleep architecture is to greatly reduce REM and slow wave sleep, leaving predominantly the lighter stages of sleep (1 and 2), as well as reducing total sleep time and sleep efficiency. However, other causes exist for deficits in sleep stages. One of the most common is medication of many sorts, especially antidepressants, which tend to suppress REM sleep.
Another abnormality to look for if you are suspected of having narcolepsy is the occurrence of the first REM sleep much sooner after falling asleep than the usual 90-minute delay. Some shortening of REM delay or "latency" can represent a sign of depression, but this does not usually cause the same degree of shortening as narcolepsy.
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