**1. Introduction**

Electrophysiological signal recordings are used in medicine for research, clinical diagnosis and follow-up of diseases as well as for providing guidance to their treatment. For example; "Electrocardiogram (ECG)" is used daily as inpatient

and as outpatient, it is the most basic electrophysiological signal recording in which five waves (P, Q, R, S, and T) are interpreted. When all monitorization activities performed at the bedside of the patient are taken into consideration, recording electrophysiological signals with well-calibrated equipment and correct interpretation of the obtained results by doctors and healthcare staff seems to be at the crossroads of correct diagnosis, follow-up and treatment. In sleep monitorization, electrophysiological signal recordings are performed by multiple electrodes and provide us with important clinical information. In fact, monitoring wakefulness as much as sleep is quite important in clinical practice; it helps to establish a correct diagnosis in clinical practice and sometimes provides the opportunity to have access to unsuspected information. If PSG could be used as frequently as ECG by well-trained medical doctors and healthcare personnel in sleep medicine, sleep health and sleep disorders of the individuals in the society could be understood much better. Therefore, health could be evaluated not only in wakefulness but also in sleep leading to a continuum. During its preliminary years, sleep related studies attracted the attention of physiologists and as time passed clinical information regarding sleep disorders increased significantly and the possibility to treat all these diseases brought the attention of clinicians into this field. For human physiology and especially for the central nervous system to continue its functioning; there needs to be a healthy interaction and an organism specific balance between wakefulness and sleep cycles. Sleep is a physiological need; a state where the response of the brain to environmental stimuli has stopped reversibly. The insufficiency or absence of this need negatively influences the interactions in the neuronal circuits and pathways that are responsible for the wakefulness of the brain. It is very well known that many functions of the organism change during sleep and different physiological mechanisms come into play during NREM and REM sleep. Diseases also show changes during sleep and during NREM and REM phases. Electrophysiological studies could assist in the understanding of basic mechanisms in neurological sciences. Electrophysiological methods and PSG that are geared to understand the nights as well as the days aim at not only establishing correct diagnosis and delineating pathophysiological mechanisms but also engaging in innovation and developing novel diagnostic and therapeutic methods.
