Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
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An EEG rhythm with a frequency of 8-13 Hz in human adults, which is most prominent over the parieto-occipital cortex when the eyes are closed. The rhythm is blocked by eye opening or other arousing stimuli. It is indicative of the awake state in most normal individuals. It is most consistent and predominant during relaxed wakefulness, particularly with reduction of visual input. The amplitude is variable, but typically is below 50 µV in the adult. The alpha rhythm of an individual usually slows by 0.5-1.5 Hz and becomes more diffuse during drowsiness. The frequency range also varies with age; it is slower in children and older age groups relative to young and middle-aged adults.