General aspects

Overnight sleep is not necessarily the quiescent state commonly supposed. There are many physiological processes and behaviours which are initiated or accentuated in relation to sleep and these occur in many people of all ages. Unfortunately, when medical advice is sought for such events, considerable confusion seems to exist about their precise diagnosis and about their psychological or physical significance. These diagnostic uncertainties are often the result of insufficiently detailed descriptions and a general lack of awareness of the many different types of nocturnal disturbance. Uncertainty about the possible significance of the episodes is largely the result of inadequate research.

Definitions and classification

In the ICSD, parasomnias are defined as episodic disorders of arousal, partial arousal, or sleep-stage transition, involving activation of the central nervous system, which intrude into sleep. They are divided into the following four groups according to the stage of sleep with which they are usually associated:

• arousal disorders (usually arising from deep NREM sleep);

• episodes which occur in the transition between wakefulness and sleep;

• parasomnias usually associated with REM sleep;

• other parasomnias which do not fall into these categories and can be associated with various sleep stages.

Parasomnias are classified separately from the dyssomnias (disorders involving difficulty initiating or maintaining sleep, or of excessive sleepiness) and also from mental, neurological or other medical conditions in which sleep disturbance or excessive sleepiness is a major feature.