Description of healthy and disturbed sleep by means of the spectral frequency index (SFx) - a retrospective analysis.

OBJECTIVES: Aim of the study was to describe the sleep of patients with sleep disorders by means of the spectral frequency index (SFx) in comparison to healthy subjects.
METHODS: Polysomnographic data of two consecutive nights recorded in healthy subjects (n = 18), patients with insomnia (n = 19), snoring (n = 29) and obstructive sleep apnoea (n = 28) were analysed retrospectively by using an automatic sleep staging system and the EEG-derived SFx. The SFx data were related to the factor values of the sleep questionnaire SF-A.
RESULTS: It was possible to describe the sleep intensity and stability and to document clear differences between healthy and disturbed sleep by means of the SFx. For all included subjects the time spent in middle deep sleep (SFx-range Z(D)) seemed to be of great importance for the subjective assessment (SF-A) of "sleep quality". A great variability within the range of light sleep was related to a negative assessment of "sleep quality" and "feeling of recovery after sleep", while greater "psychic balance in the evening" before sleep was related to a reduced variability in the range of light sleep. A sleep quality index derived from the SFx-values is described which correlates with the subjective estimation of disturbed sleep in insomnia and apnoea patients.
CONCLUSIONS: The main advantage of this method in comparison to the visual sleep scoring system is that the depth of sleep is assessed objectively and described continuously.