During which sleep phase is the body's physiological activity generally reduced?

Non-REM sleep is characterized by a reduction in physiological activity, which includes a decrease in heart rate, breathing rate, and overall brain activity. Throughout the stages of non-REM sleep—particularly in deep sleep, also known as slow-wave sleep—there is a significant drop in metabolic activity, making this phase restorative for the body.

During non-REM sleep, the body enters a state where it is less responsive to external stimuli, allowing for crucial processes such as tissue repair and growth to occur. This contrasts sharply with REM sleep, during which brain activity is heightened and the body experiences rapid eye movements, with dreams typically occurring. The awake state has high physiological activity, and the hypnagogic state, which refers to the transitional phase between wakefulness and sleep, does not represent a stable sleep phase with consistent physiological reduction. Thus, non-REM sleep is the stage where the body’s physiological activity is most reduced.