The second stage of sleep includes ________ or short runs of rhythmical responses of 12 to 16 hertz.

A) rapid eye movement (REM)
B) spindles
C) delta waves
D) gamma waves

The second stage of sleep is characterized by spindles, which are short runs of rhythmical responses of 12 to 16 hertz. REM sleep occurs later in the sleep cycle and is characterized by rapid eye movements. Delta waves are typically seen in deep sleep, which occurs in later stages of the sleep cycle. Gamma waves are high-frequency brain waves associated with cognitive processing and are not typically associated with sleep.