This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Physiology
"Theta waves" are seen in 
Select and explain. 
A. Stage 1 sleep, B. Stage 2 sleep, C. Stage 3 sleep, D. Stage 4 sleep
A: Stage 1 sleep
Answer: A: Stage 1 sleep 
Explanation: 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