NREM sleep - Mixed sleep–wake states

Intrusion of REM into NREM sleep does not cause any symptoms because sleep is maintained, but is very common. Muscle atonia, which is characteristic of REM sleep, is seen in NREM sleep, especially just before and after REM sleep episodes. The peripheral vasoconstriction of REM sleep also occurs up to 30min before REM sleep episodes, and penile erections are common at times when a REM sleep episode would be anticipated, even if its electroencephalogram features are not sufficient for it to be identified by conventional scoring systems.

The narrative content of dreams in NREM sleep is also greatest when they arise close to a previous episode of REM sleep, suggesting that REM sleep mechanisms are responsible.

All these features suggest that fragments of REM sleep are capable of intruding into what is conventionally classified as NREM sleep at times when there is pressure to enter REM sleep.

Intrusion of wakefulness is characteristic of 'disorders of arousal'. In these the transition from NREM sleep to wakefulness is incomplete or gradual and features of both states coexist temporarily. The commonest and mildest example is sleep inertia, in which sleep is followed by drowsiness or at least a sensation of feeling unrefreshed and of being no more alert after sleep than beforehand. This is often combined with temporary disorientation. The electroencephalogram shows some features of wakefulness and of stages 1 and 2 NREM sleep.

There is probably a spectrum of the degree of failure to arouse from NREM sleep which leads from sleep inertia through confusional arousals to activities such as sleep walking and sleep terrors.

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REM sleep

Intrusion of wakefulness into REM sleep is responsible for sleep paralysis at the end of a period of sleep.
Awareness of the environment is accompanied by the motor inhibition of REM sleep. Lucid dreaming, in which there is awareness of dreaming and the ability to direct its content, reflects the combination of awareness typical of wakefulness, with continuing dream mentation. Retention of muscle 'tone' in the REM sleep behaviour disorder is dissociated from the continuing REM sleep dream mentation with the result that cerebral activities are physically enacted, unlike normal sleep in which they are suppressed by motor inhibition.

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Fig. 1.3 Patterns of sleep.