Final answer:
Stage 3 sleep is marked by delta waves, which are characterized by low frequency and high amplitude. These waves are distinct from alpha and beta waves found in other sleep stages and indicate a state of deep or slow-wave sleep, where it is harder to wake a person.
Step-by-step explanation:
Stage 3 sleep is marked by delta waves. These waves are characterized by their low frequency (less than 3 Hz) and high amplitude, distinguishing them as the brain wave patterns with the lowest frequency and highest amplitude seen during sleeping states. This stage of sleep, often referred to as deep sleep or slow-wave sleep, involves a significant decrease in heart rate and respiration, making it more difficult to awaken someone from this phase. It also coincides with a decrease in muscle tension and core body temperature.
Delta waves are distinct from the alpha waves of the early NREM stage 1 sleep, which are lower in frequency (8-12 Hz) and higher in amplitude than the beta waves characteristic of wakefulness (with a frequency of 13-30 Hz). Similarly, theta waves are associated with the latter part of stage 1 sleep and have even lower frequency (4-7 Hz) and higher amplitude than the alpha waves. However, it's the delta waves that are the hallmark of stage 3 sleep.
Furthermore, sleep spindles and K-complexes, which are indicative of stage 2 sleep, do not appear in stage 3. In fact, an increased level of alpha brain wave activity during stage 3, which is not typical of this deep sleep phase, can result in a feeling of non-restfulness upon waking.