"Theta waves" are seen in
Stage 1 sleep
Stage 2 sleep
Stage 3 sleep
Stage 4 sleep
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
Nervous system