**Core Concept**
During sleep, the human brain cycles through different stages characterized by distinct electroencephalogram (EEG) patterns. Stage 2 non-rapid eye movement (NREM) sleep is marked by specific EEG features, including K-complexes and sleep spindles, which are crucial for memory consolidation and regulation of the sleep-wake cycle.

**Why the Correct Answer is Right**
K-complexes and sleep spindles are characteristic EEG patterns observed during stage 2 NREM sleep. K-complexes are high-amplitude, negative-deflection waves that occur in response to external stimuli, serving as a mechanism to prevent sleep disruption. Sleep spindles, on the other hand, are short bursts of rapid, low-amplitude waves that help to filter out background noise and maintain sleep. These features are essential for the proper functioning of stage 2 NREM sleep.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because stage 1 NREM sleep is characterized by a gradual transition from wakefulness to sleep, with a mix of alpha and theta waves on the EEG.
**Option B:** This option is incorrect because stage 3 NREM sleep, also known as slow-wave sleep, is marked by high-amplitude delta waves, which are not associated with K-complexes and sleep spindles.
**Option C:** This option is incorrect because REM sleep is characterized by rapid eye movements, low-amplitude, high-frequency waves on the EEG, and vivid dreams, which are distinct from the EEG patterns observed in stage 2 NREM sleep.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize the different stages of sleep and their corresponding EEG patterns to diagnose sleep disorders, such as insomnia or sleep apnea, and to understand the underlying mechanisms of sleep regulation.

**Correct Answer:** D. Stage 2 NREM sleep.