Final answer:
The stages of sleep are characterized by distinct patterns of brain activity, from alpha and theta waves in the initial Non-REM stages to delta waves in deep sleep, and REM sleep where brain activity resembles an awake state.
Step-by-step explanation:
The stages of sleep are delineated by patterns of brain activity. During the different stages, our brain waves vary significantly, which can be observed using an Electroencephalogram (EEG). As we transition from wakefulness to sleep, high-frequency beta waves are replaced by alpha and then theta waves in the first stage of Non-REM sleep.
This is followed by sleep spindles and K-complexes in stage 2, and low-frequency, high-amplitude delta waves in stage 3, also known as slow-wave sleep. REM sleep, on the other hand, is characterized by rapid eye movements and brain waves that closely resemble those of an awake state. Both Non-REM and REM sleep are essential for various cognitive functions, including learning and memory.