**Core Concept:** Bruxism is a clinical condition characterized by involuntary grinding or clenching of teeth, typically occurring during sleep. It can lead to tooth wear, jaw pain, and TMJ disorders.

**Why the Correct Answer is Right:** Bruxism mainly occurs in the **Rapid Eye Movement (REM) sleep phase**. This is the phase when dreams occur and the brain is most active. During REM sleep, increased muscle activity, including teeth clenching, results in bruxism.

**Why Each Wrong Option is Incorrect:**
1. **Option A (Non-REM sleep):** Non-REM sleep includes stages 1 to 3, which are characterized by reduced muscle activity and minimal eye movement. Therefore, bruxism is unlikely to occur in this phase.
2. **Option B (Wakefulness):** As bruxism is a sleep-related condition, it does not occur in the wakefulness phase, which is characterized by full consciousness and lack of muscle activity.
3. **Option C (Transition to REM sleep):** Although bruxism can start during the transition phase, it predominantly occurs in the REM sleep phase, making it the correct answer.

**Clinical Pearl:** Although bruxism is a common sleep disorder, it's essential to differentiate between these sleep phases to understand and diagnose the correct condition. A thorough sleep study can help identify the specific sleep phase with bruxism, guiding appropriate treatment strategies.

**Correct Answer:** Rapid Eye Movement (REM) sleep