Changes in frequency and amplitude of delta activity during sleep

Modified period analysis was applied to all-night sleep recordings from 14 young adult males. The modifications involved addition of measures of integrated amplitude and of time in frequency band to the zero crossings and zero counts of the first derivative. The analysis was directed toward changes in the characteristics of delta activity (0--3 c/sec) across the night. Delta shifted toward lower frequencies and decreased in amplitude as sleep progressed. These trends were apparent in mean values for successive periods of slow-wave sleep (SSWPs). For epochs of record classified as stage 4 EEG, these trends were seen both within and across SSWPs. The physiological significance of these changes is unknown. We suggest that they may reflect the kinetics of the metabolic processes underlying sleep. Whatever their theoretical significance, the present results provide new, normative features of the sleep EEG which are not evident in sleep stage classification. The trends we observed appear sufficiently reliable to merit empirical investigation as a function of age, clinical state, or the administration of drugs in addition to further studies aimed at determining their biological significance.