Question

Stage 2 sleep is defined by the presence of

• A. \( \text{spikes and slow waves} \)
• B. \( \text{sleep spindles and K complexes} \)
• C. \( \text{rapid eye movements} \)
• D. \( \text{1 to 2 Hz delta frequencies} \)

Hint:

To remember the defining features of sleep stages, focus on the unique EEG patterns:

• N1 (Drowsiness) Theta waves, vertex sharp waves, slowing of alpha rhythm.
• N2 (Light Sleep) **Sleep spindles** and **K complexes** are the defining features.
• N3 (Deep/Slow-Wave Sleep) Predominance of high-amplitude **delta waves** (slow waves).
• REM (Dreaming Sleep) Low-voltage, mixed-frequency EEG (similar to awake state), **rapid eye movements**, and muscle atonia.

Understanding these distinct EEG patterns is fundamental for sleep staging in polysomnography.

Answer 1

The Correct Option is B

Sleep is generally divided into two main stages: Rapid Eye Movement (REM) sleep and Non-Rapid Eye Movement (NREM) sleep. NREM sleep is further subdivided into three (or four, depending on the classification system) stages: N1, N2, and N3 (or N3 and N4 for deep sleep). Let's examine the characteristics of each option in relation to sleep stages:

• (A) spikes and slow waves: "Spikes" alone are not a defining feature of a normal sleep stage. "Slow waves" are characteristic of deeper NREM sleep, specifically Stage N3 (also known as slow-wave sleep or SWS), which is defined by the presence of high-amplitude, low-frequency (0.5 to 2 Hz) delta waves.
• (B) sleep spindles and K complexes: These are the hallmark EEG phenomena that define Stage 2 (N2) sleep.
  • Sleep spindles are bursts of oscillatory brain activity in the 12-14 Hz range, typically lasting 0.5 to 1.5 seconds. They are thought to be involved in memory consolidation and protecting sleep from external disturbances.
  • K complexes are high-amplitude, biphasic (negative followed by positive) waves lasting around 0.5 seconds. They can occur spontaneously or in response to external stimuli and are believed to play a role in arousal suppression and memory consolidation.
• (C) rapid eye movements: Rapid eye movements are the defining characteristic of REM sleep. This stage is also associated with muscle atonia (paralysis) and vivid dreaming.
• (D) 1 to 2 Hz delta frequencies: These low-frequency, high-amplitude delta waves are characteristic of Stage N3 (slow-wave sleep or deep sleep). While some delta activity can be present in N2, it is the predominant feature of N3. The option specifies 1 to 2 Hz, which falls squarely within the delta wave range.

Therefore, the presence of sleep spindles and K complexes uniquely defines Stage 2 (N2) sleep.