**Core Concept:** Electroencephalography (EEG) is a non-invasive technique used to record electrical activity of the brain. During sleep, the brain exhibits distinct patterns that can be detected by an EEG. There are five stages of sleep, each with unique EEG characteristics.

**Why the Correct Answer is Right:** Stage 3 of non-rapid eye movement (NREM) sleep is characterized by slow wave activity, also known as delta waves. These waves are present in EEG recordings during this stage and are responsible for the distinctive pattern seen in the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Delta waves are not present in stage 1 sleep, which is characterized by the presence of spindles and K-complexes.
B. Stage 2 sleep is characterized by the presence of high-frequency waves, also known as sharp waves and الموجات of rapid eye movement (REM) sleep.
C. Stage 3 sleep is the correct answer, as it is the stage associated with delta waves in the EEG recording.
D. Stage 4 sleep, also known as deep sleep, is characterized by increased delta waves and is distinct from stage 3 sleep.

**Clinical Pearl / High-Yield Fact:** The presence of delta waves in EEG recordings is crucial for diagnosing sleep stages, particularly in cases of suspected sleep disorders or neurological conditions. Understanding sleep stages and their corresponding EEG patterns is essential for accurate diagnosis and treatment planning.

**Correct Answer:** D. Stage 4 sleep (Delta waves)