Sleep has many important parameters, for example, the total duration of sleep, maintenance of sleep, and at what time of the day we fall asleep. Electrophysiologically, sleep may be divided into two main stages: non-rapid-eye- movement (NREM) sleep and rapid-eye-movement (REM) sleep. NREM sleep can be further divided into three stages: N1, N2, and N3. Sleep is dependent upon two processes: homeostatic and circadian. Sleep starts with NREM sleep, and after approximately 90 min, the first episode of REM sleep appears. REM sleep is characterized by low voltage mixed frequency activity and sawtooth waves in Electroencephalogram (EEG). REM sleep is modulated by laterodorsal tegmental/ pedunculopontine tegmental (LDT/PPT) cholinergic nuclei situated in the brainstem. Sleep is regulated by the circadian as well as the homeostatic processes. Throughout the sleep, neuronal activity keeps changing, which is responsible for the generation of various physiological characteristics that help to determine the different sleep stages.