In sleep research, what does "sleep phase" refer to?

The term "sleep phase" specifically refers to the distinct periods of sleep that are characterized by unique patterns of brain activity and physiological changes. These phases include non-REM (Rapid Eye Movement) sleep, which has three stages (N1, N2, N3), and REM sleep. Each of these phases serves different functions in the overall sleep cycle, such as physical restoration, memory consolidation, and emotional regulation. The specific brain waves, muscle tone, and other physiological markers are used to classify which phase a person is currently in during their sleep cycle. Understanding these phases is crucial for sleep research, as they relate to the quality of sleep and overall health.