Which pattern of high amplitude brain activity is associated with stage 2 sleep?

• A. Alpha waves
• B. Sleep spindles
• C. Theta waves
• D. K complexes

Answer: (D)

Stage 2 sleep is defined on EEG by two features, including large, high-amplitude waveforms called K-complexes. A K-complex is a single, prominent sharp deflection (often followed by a positive wave) that can occur spontaneously or in response to a stimulus, lasting about half a second to a couple of seconds. This high-amplitude pattern is a hallmark of stage 2, signaling a shallow progression into deeper sleep and helping protect sleep from brief awakenings.

Alpha waves describe relaxed wakefulness with eyes closed, so they don’t fit stage 2. Theta waves are characteristic of lighter sleep, such as stage 1. Sleep spindles are bursts of 12–14 Hz activity that occur during stage 2 but are shorter and not typically described as high amplitude the way a K-complex is. So the high-amplitude pattern associated with stage 2 is the K-complex.