Final answer:
People in stage 4 sleep, which is deep or slow-wave sleep marked by delta waves, are hard to wake up. Sleep spindles and K-complexes are characteristic of stage 2 sleep, which is a state of deep relaxation.
Step-by-step explanation:
People in stage 4 sleep are generally hard to wake up. This phase of sleep is also known as slow-wave sleep or deep sleep and is characterized by the presence of low-frequency, high-amplitude delta waves. During this stage, there is a significant decrease in heart rate and respiration, and the body is less responsive to external stimuli, making it more difficult to awaken a person than during the earlier stages of sleep.
As for sleep spindles and K-complexes, they are most often associated with stage 2 sleep. Sleep spindles are rapid bursts of high frequency brain waves that may be important for learning and memory. Stage 2 sleep is a state of deep relaxation that precedes the deeper stages of slow-wave sleep, which encompass stages 3 and 4.