Alpha wave

Alpha waves describe a type of brain activity that appears on an electroencephalogram. They range in frequency from 7.5 to 12.5 cycles per second and original from electrical activity in the thalamus and the occipital lobe. Although their amplitude (height) is reduced in those who are drowsy or asleep, during sleep they become the most dominant of the measured activity on an EEG. They are named "alpha", the first letter of the Greek alphabet, as they were the first type of brain wave to be discovered and measured. 
Alpha wave at Wikipedia