**Core Concept**
Sleep spindles are a characteristic feature of non-rapid eye movement (NREM) sleep, specifically stage 2 NREM sleep. They are short bursts of brain activity that occur when the brain switches from a state of low voltage to a state of high voltage, resulting in a characteristic "spindle-like" pattern on the electroencephalogram (EEG).

**Why the Correct Answer is Right**
During stage 2 NREM sleep, the body's temperature drops, heart rate slows, and the brain waves slow down. The EEG pattern changes to show bursts of 12-16 Hz brain waves, which are known as sleep spindles. These spindles are thought to be important for memory consolidation and learning. They are generated by the synchronization of neural activity between different parts of the brain, particularly between the thalamus and the cortex.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because sleep spindles are not characteristic of stage 3 NREM sleep, which is also known as slow-wave sleep. Stage 3 NREM sleep is characterized by slow delta waves, not the fast, synchronized activity of sleep spindles.
**Option B:** This option is incorrect because rapid eye movement (REM) sleep is a distinct stage of sleep that is characterized by rapid eye movements, low voltage on the EEG, and high brain activity. Sleep spindles are not a feature of REM sleep.
**Option C:** This option is incorrect because stage 1 NREM sleep is the lightest stage of sleep and is characterized by a gradual decrease in consciousness and a slowing of brain waves. Sleep spindles are not typically seen in stage 1 NREM sleep.

**Clinical Pearl / High-Yield Fact**
Sleep spindles are an important feature of stage 2 NREM sleep, and their presence can help diagnose sleep disorders such as insomnia or sleep apnea. Additionally, sleep spindles are thought to play a role in memory consolidation, so disruptions in sleep spindles may have implications for cognitive function.

**Correct Answer: C. Stage 1 NREM sleep.**