Quantitative description of EEG periodicities during stationary sleep stages

A quantitative method was applied in order to assess variations in EEG activities during sleep. Three classes of variations were distinguished: variations connected with sleep-stage changes (class 1), higher-frequency variations described by the envelope of frequency-band activities (class 2), intermediate-frequency variations, corresponding to periods from 4 to 120 s (class 3). For each class, parameters characterizing the frequency spectra were computed. These parameters were mathematically simple and clear in their meaning, since they measured power, modulation index and mean squared frequency. A statistical comparison of the mean values of the parameters during different sleep stages evidenced a certain number of significant shifts in each of the three classes. The most important class-1 and class-2 variations were described by our parameters with high levels of significance. The results obtained for class 3 were in agreement with visual observations reported in the literature, such as the progressive increase in the interval between successive arousals from sleep onset to Stage 4 and the frequent occurrence of cyclic alternating patterns during Stage 2 epochs immediately preceding REM sleep.