Which sleep stage is characterized by decreased respiration, heart rate, body temperature, muscle tension, and is easy to wake?

The stage identified as Non-rapid Eye Movement (NREM)1 is characterized by a transition between wakefulness and sleep. During this stage, physiological signs such as respiration and heart rate begin to slow down, and body temperature starts to drop. Muscle tension decreases, and individuals in this stage can be easily awakened, often without significant disruption to their consciousness.

This stage is also marked by the ability to experience light sleep, where individuals might even perceive external stimuli or suddenly wake up as if they were only momentarily dozing off. The other stages of sleep, particularly the deeper stages, exhibit more pronounced changes in respiration and heart rate, with a more substantial decrease in awareness and difficulty waking. NREM3, often considered deep sleep, is associated with even greater physiological stability and reduced responsiveness to external stimuli, making awakening less likely and more difficult. Thus, the characteristics of NREM1 make it the correct choice for this question.