Other equipment used in support with EEG, is one that measures eye movements that is called electrooculogram or EOG. The Electromyogram (EMG) on the other hand measures the muscle tension under the chin of a subject. As stated in Sleep-Information about Sleep, the practice in studying sleep pattern is EEGs, EOGs, and EMGs are recorded simultaneously on a continuous moving chart paper. This then determines the activities of three aspects – the muscle activity, the brain activity, and eye movements.
The NREM sleep and REM sleep occurs in a cyclic manner during sleep state (Sleep, Sleep Disorders, and Biological Rhythms. They interchangeably happen to an individual during his sleep. Sleep starts with periods of NREM and then REM. NREM state has four different stages as determined by the frequency and amplitude (brain wave’s magnitude of periodic variation) of the brain activity. The first NREM stage is characterized by very light sleep. NREM Stage 2 has special brain waves called sleep spindles (burst of brain activity visible on an EEG) and K-complexes (brief high-voltage peak in EEG as defined by Wikipedia.org). Stage 3 and 4 of NREM is described by slow brain waves and in Stage 4 of NREM, it is hard to wake the person up. NREM stage 4 is deep sleep and cannot easily be awakened by external factors. During NREM, the muscle activity remains active and body function is the same with the waking state. The difference is that there is no eye movement.