Which phenomenon is described as sudden, brief muscle contractions that can wake a person from light NREM sleep?

• A. Hypnic jerk
• B. Alpha waves
• C. Delta waves
• D. Sleep spindles

Answer: (A)

A hypnic jerk is a sudden, brief muscle contraction that often occurs as you’re transitioning from wakefulness into sleep, and it can wake you from light NREM sleep. It’s a common, benign sensation—sometimes described as a jolt or a falling feeling—that happens during the moment your body relaxes into sleep. The other terms refer to brain activity rather than muscle twitches: alpha waves show relaxed wakefulness with closed eyes, delta waves appear in deep slow-wave sleep, and sleep spindles are bursts of activity typical of stage 2 sleep. None of those describe a sudden muscle contraction that can wake you, which is why this phenomenon best fits the description.