Sleep stages are different stages of sleep, separated from each other basically by brain activity. In addition to brain activities in EEG, other components of polysomnography such as EOG (measurement of eye movements) and EMG (measurements of body muscle activity) are used to define and monitor the stages. Sleep stages are needed in order to be able to quantitatively evaluate the sleep state, which can be quite variable, and to show a more accurate approach in the identification of diseases.