Sleep EEG in the sigma and delta frequency bands was subjected to spectral analysis in 8 normal young adults. In each subject, power density of sigma and delta oscillated reciprocally during NREM sleep, confirming an observation made initially with period/amplitude analysis. In REM sleep, power density for both frequency bands was at its lowest levels. Correlation coefficients between power density of delta vs. 1/sigma for all artifact-free 20-s epochs of NREM sleep/night were highly significant for each subject. These results show that cyclic oscillation of EEG within sleep is not limited to delta frequencies. The reciprocal relation of sigma to delta holds implications for the EEG mechanisms of NREM sleep. This dynamic pattern may also prove useful for sleep stage scoring and for a finer empirical analysis of sleep in psychiatric and neurological disorders.