Age and gender affect different characteristics of slow waves in the sleep EEG

Low-frequency EEG was analyzed quantitatively during 2 nights in 40 females and 34 males aged 26 to 101 years. Analyses were based on Rechtschaffen and Kales NREM sleep stages, on absolute low-frequency amplitude (i.e. power in the range of 0.2-2.0 Hz) and on low-frequency continuity. The latter parameter describes how much (0-100%) of the current slow-wave activity is continued in the near-future EEG. Such continuation can occur through closed loops in the underlying neuronal network and cells. These loops are slow, thus corresponding to slow-wave frequencies, and can consist of electrophysiological, chemical and/or other pathways. The continuity percentage then monitors the relative activity of these loops. It does not depend directly on absolute EEG amplitudes. All analyzed parameters, including amplitude-independent continuity, decreased substantially and significantly with increasing age. The amplitudes of low-frequency EEG in females were significantly and substantially (40%) larger than those in males. However, the amplitude-independent continuity percentage did not differ between the genders. These findings support the notion that gender-related anatomical differences have a general effect on EEG amplitude, including during slow-wave sleep. Aging, however, specifically affects the neurophysiological slow-wave-generating mechanism.