Which EEG finding is indicative of sleep stage R?

The presence of sleep spindles along with rapid eye movements (REM) is characteristic of sleep stage R, also known as REM sleep. During this stage, the EEG typically shows mixed frequency activity, which may include theta waves, but the hallmark of REM sleep is the presence of these rapid eye movements and the sleep spindles. Sleep spindles are bursts of oscillatory brain activity that occur predominantly during NREM sleep, yet in the context of REM sleep, they can still be present, often indicating transitions between brain states.

In addition to spindles, the activation in REM sleep is also linked with muscle atonia, which is often recorded as chin EMG activity during polysomnography, allowing for the identification of the hypnogram where sleep stage R is detected. Electric activity during this phase is characterized by a low amplitude mixed frequency pattern, which helps distinguish it from other sleep stages.

The other options do not correctly identify the unique features of sleep stage R. Delta waves are primarily associated with deeper stages of sleep (NREM), theta waves can appear in NREM but are not indicative of REM sleep without the context of rapid eye movements, and alpha activity typically represents an awake state or relaxed wakefulness not aligned with the features