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Functional sleep disturbance (or functional hyposomnia) is the most frequent and therefore the most important form of deviation from healthy sleep. This term applies to a group of sleep disturbances in which the disturbance is functional—i.e., is not the result of an exogenous (resulting from external causes) or of an endogenous (having its origin in the body) illness. The term functional hence expresses something about the function, meaning that the structure of the function is disturbed without directly making any statement about the cause (etiology). Frequently the interplay of various regulatory mechanisms is disturbed at the same time without the reason being known.

Neurologically the combined symptoms of the disturbance of function that subsequently leads to functional sleep disturbance probably consists in the disturbed course of the synchronic and asynchronic sleep phases. Instead of the normal cyclical change, a rapid oscillation (phase change) occurs between different deep sleeping and waking periods where there is altogether too little deep sleep (Baust).

These findings explain why so many persons suffering from functional sleep disturbances have the feeling of not having slept at all, even when they did objectively sleep. The impossibility of the individual's precise assessment often makes diagnosis significantly more difficult.

The difficulty already mentioned of a person's judging himself to be still awake or already asleep was documented by Baust (1967) in very clear observations made concerning sleep experiments made in a laboratory: "Many subjects assert upon awakening from the stage of sleep onset or from light sleep that they were wide awake and had been thinking about something. Consequently, in the stages of light sleep it is impossible to make precise statements about the degree of wakefulness.

"Similarly, a group of subjects asserted upon awakening from dream stages that they had been wide awake. Dreams, too, are often interpreted as thoughts experienced while awake. The explanation for a patient's assertion of utter sleeplessness is consequently the result of more light sleep stages or the false assessment of dream phases."

Clinical and neurophysiological evidence has strengthened my view that the subjective feeling, "I have slept," that every normal sleeper has upon awakening is evidently coupled with the periodic alternation between synchronic and asynchronic sleep phases. If this alternation is missing, as for example in the constant oscillation between light and deep sleep of the person suffering from functional sleep disturbance, the subjective feeling of having slept is also absent. This again shows the close connection between physical feeling and psychic assessment—i.e., the somatopsychic aspect from which sleep and especially functional sleep disturbance must be considered.

The course of such functional sleep disturbances is definitely chronic. Severe forms of sleep disturbances often begin as early as in the third decade of life, or between age 20 and 30, and then manifest a slow and constant increase. Only rarely is the pattern of development one of periods of worsening, alternating with periods of remission.