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Abstract
Low-frequency (< 1 Hz) oscillations in intracellular recordings from cortical neurons
were first reported in the anaesthetized cat and then also during natural sleep. The
slow sequences of hyperpolarization and depolarization were reflected by slow oscillations
in the electroencephalogram. The aim of the present study was to examine whether comparable
low-frequency components are present in the human sleep electroencephalogram. All-night
sleep recordings from eight healthy young men were subjected to spectral analysis
in which the low-frequency attenuation of the amplifier was compensated. During sleep
stages with a predominance of slow waves and in the first two episodes of non-rapid-eye-movement
sleep, the mean power spectrum showed a peak at 0.7-0.8 Hz (range 0.55-0.95 Hz). The
typical decline in delta activity from the first to the second non-rapid-eye-movement
sleep episode was not present at frequencies below 2 Hz. To detect very low frequency
components in the pattern of slow waves and sleep spindles, a new time series was
computed from the mean voltage of successive 0.5 s epochs of the low-pass (< 4.5 Hz)
or band-pass (12-15 Hz) filtered electroencephalogram. Spectral analysis revealed
a periodicity of 20-30 s in the prevalence of slow waves and a periodicity of 4 s
in the occurrence of activity in the spindle frequency range. The results demonstrate
that distinct components below 1 Hz are also present in the human sleep electroencephalogram
spectrum. The differences in the dynamics between the component with a mean peak value
at 0.7-0.8 Hz and delta waves above 2 Hz is in accordance with results from animal
experiments.