Although accessory electrodes are commonly used for EOG and EMG recording in polysomnography (PSG) studies, the different phases of sleep are brain states, defined by the activity of the brain stem, diencephalon, and cerebral cortex. Therefore, scalp recording is sufficient for an adequate study of sleep in humans. The different phases of sleep exhibit highly specific spectral and timing features, allowing their use for sorting without the need for any accessory channels. The numerical properties of K-complexes and sleep spindles, which are waveforms of great importance in sleep pathology, are also described. NREM sleep is a continuous process and it is difficult to justify the difference between N2 and N3 phases. A couple of examples of the importance of identifying sleep phases are shown.