The brain is very metabolically active, and has a high demand for oxygen and glucose. Stopping the blood supply to the brain for as little as 4 minutes can cause irreversible brain damage The CNS is protected by a blood-brain barrier that regulates substances entering the brain. An EEG is often used as a legal criterion for death Brain waves are classified as alpha, beta, theta, and delta waves. Alpha waves are seen when the subject is awake and resting, but with eyes closed, and mind wandering Beta waves are seen during mental activity and sensory perception. Theta waves are normal in children and sleeping adults, but signal brain disorders and emotional stress in awake adults.
Delta waves are present in awake infants and in deeply sleeping adults.
If present in awake adults, they indicate brain damage Sleep Sleep is a temporary state of unconsciousness from which the person can be aroused. Stage 3, sleep deepens, vital signs decline, and theta waves appear. The person is deeply asleep. It is also called paradoxical sleep because of the difficulty with which a sleeper can be aroused. Most dreams occur during this period.