What effect on the brain wave pattern does hyperventilation have?

Hyperventilation decreases brain wave frequency

Hyperventilation (rapid breathing) leads to hypocapnia (decreased blood carbon dioxide levels). CO2 is needed for optimal neuronal activity. Lowered PCO2 decreases cerebral blood flow and the production of energy which reduces electrical activity recorded as the amplitude and the frequency of the EEG signals, resulting in slower frequency brain waves including more delta and theta activity.