**Core Concept**
The assessment of daytime functioning as an index of sleep adequacy involves evaluating the impact of sleep on a person's daily activities, cognitive performance, and mood. This concept is crucial in diagnosing and managing sleep disorders, such as insomnia, sleep apnea, and restless leg syndrome.

**Why the Correct Answer is Right**
The correct answer is related to the Pittsburgh Sleep Quality Index (PSQI), a widely used self-report questionnaire that assesses sleep quality and daytime functioning over the past month. The PSQI evaluates seven components: subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbances, use of sleep medication, and daytime functioning. These components provide a comprehensive picture of a person's sleep quality and its impact on daily functioning. The PSQI is a reliable and validated tool that helps healthcare professionals diagnose sleep disorders and monitor treatment effectiveness.

**Why Each Wrong Option is Incorrect**
**Option A:** The Epworth Sleepiness Scale (ESS) is a self-report questionnaire that measures daytime sleepiness, but it does not assess sleep quality or daytime functioning as comprehensively as the PSQI.

**Option B:** The Multiple Sleep Latency Test (MSLT) is a polysomnographic test that measures sleep latency and daytime sleepiness, but it does not evaluate daytime functioning or sleep quality.

**Option C:** The Actigraphy is a non-invasive tool that measures sleep-wake patterns, but it does not directly assess daytime functioning or sleep quality.

**Option D:** The Morningness-Eveningness Questionnaire (MEQ) evaluates circadian preference, but it does not assess sleep quality or daytime functioning.

**Clinical Pearl / High-Yield Fact**
When assessing sleep disorders, it's essential to use a comprehensive tool like the PSQI, which evaluates both sleep quality and daytime functioning. This helps healthcare professionals diagnose sleep disorders accurately and develop effective treatment plans.

**Correct Answer: C. Actigraphy is a non-invasive tool that measures sleep-wake patterns, but it does not directly assess daytime functioning or sleep quality.