Electroencephalographic and neurophysiological changes.

Sleep electroencephalography is the recording of the electrical activity of the brain during sleep. The resulting electroencephalogram (EEG) is a complex signal deriving from postsynaptic potentials of cortical pyramidal cells. The EEG is an important brain state indicator (allowing for the discrimination between wakefulness, sleep, seizures, coma) with specific state-dependent features. Sleep EEG activity can be recorded by placing electrodes on the surface of the scalp or directly into brain tissue (stereo-EEG or SEEG). Some basic, qualitative aspects of the EEG signal can be recognized by visual inspection. However, additional quantitative analysis is fundamental to investigate the EEG in more detail. In humans, electrodes are typically placed on the scalp according to the international 10–20 system. The basic features used to describe EEG waveforms are frequency, amplitude, and morphology. Frequency is measured as the number of cycles per second or Hertz (Hz). Amplitude is the height of each EEG wave, commonly reported in microvolts (mV), while morphology is a description of the shape of the EEG wave