How the Body Works An EEG: Brain Waves Brain waves recorded from the scalp prove that human beings never "switch off" their minds. Here three levels of arousal--sleeping, relaxing and action--all have a characteristic brain wave pattern. The machine used to monitor and record this activity is an electroencephalograph or EEG. In deep sleep, the EEG pattern produces large, slow waves. When relaxing the waves become faster. In an active situation the EEG is very dense and is described as having a low voltage and a high frequency, which produce a fast wave.