Differences in background EEG activity during REM sleep are commonly observed between individuals. But, so far, there has been no attempt to objectively quantify this phenomenon. Using this tool, ORPREM was found to vary from values found in stage Wake to values found in fairly deep non-REM sleep (1); see the Figure below.
Sleep depth in REM sleep differs substantially from non-REM sleep depth in several important ways:
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Whereas ORPNR fluctuates in each subject between high and low values throughout non-REM sleep and generally increases as the night progresses, ORPREM is fairly constant during stage REM and across the night (see last figure in Read More (2)).
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Average ORPREM across the night is generally higher than average ORPNR, but the difference varies greatly among subjects. Generally, when ORPNR is very low, ORPREM is not much higher than ORPNR (Figure). However, at higher ORPNR, the difference generally increases, and ORPREM can be fully in the awake range when ORPNR is below 1.00. (panel A).
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ORPREM is highly reproducible across 5 years of follow-up (ICC 0.79; panel D in figure), and more so than ORPNR (ICC 0.68; (1)). A preliminary study suggests that it is heritable.
Figure:
A) Relation between non-REM ORP and REM ORP in 5783 subjects of the Sleep Heart Health Study (SHHS).
B and C) One 30-second epoch of REM sleep from each of two subjects showing a wake EEG pattern in one (B, Average ORP=2.02), and a moderately deep sleep pattern in the other (C, average ORP 0.65). EMG, chin electromyogram; EEG, EEG (sec); C3=A2 and C4-A1 EEG electrodes; EOG, electrooculogram.
D) Intraclass correlation between REM ORP in the first sleep study (SHHS1) and second sleep study done 5 years later (SHHS-2) (Reference 1)
In a preliminary study, high ORPREM was associated with reduced REM time and increased REM sleep fragmentation (2). REM sleep reduction and fragmentation are associated with depression and PTSD (3-5). An association between ORPREM and a host of psychiatric disorders is therefore possible with potential implications for therapy for these disorders.
In some patients with OSA, but not in others, REM sleep is associated with long events and severe drops in SpO2. It is possible that this type of event occurs primarily in patients with deep REM sleep (low ORPREM).
Studies are needed to evaluate these important potential associations.
Relevant References: