Distribution of period-analyzed delta activity during sleep.

The distribution of delta EEG activity was evaluated during sleep in eight healthy adults. Digital period analysis, including an amplitude measurement, was used to quantify delta activity. The average percentages of delta half- and full-wave zero-crosses, mean delta power, and mean total power were computed for each nonrapid eye movement (NREM) period. The distribution of these delta measures was examined across NREM periods (NREMPs). No significant linear trends were evident in the zero-cross or power measures. Four subjects showed the highest delta power and greatest incidence of delta in the second rather than first NREMP. Regression analyses of individual delta incidence data revealed that only 62.5% of recording nights could be described with an exponential equation. Exponential regressions described 81% of delta amplitude cases. Further, delta activity in some subjects could be described by either linear or exponential regression. The findings suggest that the temporal changes of delta activity may not be entirely systematic across NREMPs and are variable across subjects.