The EEG of the sleep onset period in insomnia: a discriminant analysis

A period of rapid change in the wave components of the electroencephalogram (EEG) marks the transition from wake to sleep. Twenty-six insomniac and 28 control nights were studied in a discriminant analysis to determine whether this transitional state is modified in any way in subjects diagnosed for psychophysiological insomnia. A discriminant function was derived based on 20 insomniac and 22 normal nights. All 42 nights were correctly classified by this function. The sleep onset period, extending on the average over about 3 minutes, was characterized essentially by the beta and delta components of the EEG signal and by an activity index given by the ratio beta/delta, measured at the temporal lobe sites. Other variables included the subject's age and the magnitude of the changes occurring in the difference between activities in the right and left hemispheres. The variables contributing most to the discrimination were the activity index and beta, especially at the transitions from wake to stage 1 and from stage 1 to stage 2. The contribution of delta to the discrimination was less, but extended further in time to include stage 2 sleep. A test on the remaining six insomniac and six control nights gave a 75% classification accuracy, thus validating the derived discriminant function.