What term describes a sudden, brief muscular contraction that may wake someone from stage 1 NREM sleep?

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

Answer: (B)

As you drift from wakefulness into light sleep, your body can twitch briefly. This sudden, brief muscular contraction that may wake you is called a hypnic jerk. It commonly happens during the transition into stage 1 NREM sleep and is a normal, benign phenomenon many people experience.

Delta waves describe the slow brain activity typical of deep sleep (stages 3 and 4), not a muscle contraction. Sleep spindles are brief bursts of brain activity seen in stage 2 NREM, and K complexes are large waves also associated with stage 2; neither describes a sudden muscle jerk.