"Theta waves" are seen in
The first stage of sleep is characterized by theta waves, which are even slower in frequency and greater in amplitude than alpha waves. The difference between relaxation and stage 1 sleep is gradual and subtle. A person falling asleep first enters stage 1, the EEG begins to show a low-voltage, mixed-frequency pattern. A theta rhythm (4-7 Hz) can be seen at this early stage of slow-wave sleep. Theta and delta rhythms are normal during sleep, their appearance during wakefulness is a sign of brain dysfunction.Ref: Ganong's Review of Medical Physiology, Twenty-Third Edition
Stage 1 sleep
Stage 2 sleep
Stage 3 sleep
Stage 4 sleep
Nervous system