Which of the following stages of sleep typically has the lowest arousal threshold?

The stage of sleep known as REM (Rapid Eye Movement) typically has the lowest arousal threshold among all the sleep stages. This means that an individual in REM sleep is more easily awakened compared to those in previous stages of sleep. This stage is characterized by rapid eye movements, increased brain activity, and vivid dreams.

During REM sleep, although the body is in a state of paralysis (atonia) to prevent acting out dreams, the brain resembles its waking state in terms of electrical activity. As a result, external stimuli can more easily interrupt this stage, leading to a lower arousal threshold. This unique combination of brain activity and decreased sensitivity to external stimuli sets REM sleep apart from N2 and N3 stages, as well as the awake state, where a person is fully alert and responsive to stimuli.

In contrast, the deeper stages of sleep, such as N3, have a higher arousal threshold because they are associated with more restorative processes and less reactivity to external events. Thus, the characteristics of REM sleep confirm why it is the stage with the lowest arousal threshold.