Obstructive sleep apnea/hypopnea syndrome (OSAHS) is characterized by recurrent episodes of partial or complete upper airway collapse during sleep that is highlighted by a reduction in, or complete cessation of, airflow despite documented on going inspiratory efforts. Due to the lack of adequate alveolar ventilation that results from the upper airway narrowing, oxygen saturation may drop and partial pressure of CO2 may occasionally increase. The events are mostly terminated by arousals. Clinical consequences are excessive daytime sleepiness related to the sleep disruption. Minimal diagnostic criteria have been defined for OSAHS. Patients should have excessive daytime sleepiness that can not be better explained by other factors, or experience two or more of the following symptoms, again that are not better explained by other factors: choking or gasping during sleep; recurrent awakenings from sleep; un-refreshing sleep; daytime fatigue; and impaired concentration. All patients should have more than five obstructed breathing events per hour during sleep. An obstructive apnea or hypopnoea can be defined as an event that lasts for ≥ 10 s and is characterized by an absence or a decrease from baseline in the amplitude of a valid measure of breathing during sleep that either reaches >50% with an oxygen desaturation of 3% or an arousal (alternatively a 30% reduction with 4% desaturation). The American Academy of Sleep Medicine (AASM) recommends these definitions. The Task Force of the AASM also states that there are common pathogenic mechanisms for obstructive apnea syndrome, central apnea syndrome, sleep hypoventilation syndrome and Cheyne-Stokes breathing. It was more preferable to discuss each of these separately; although, they could be placed under the common denominator of "sleep-disordered breathing syndrome". The definition of OSAHS using two components, daytime symptoms and breathing pattern disturbances during sleep, may suggest that there is a tight correlation between the two. However, unfortunately this is not the case. The breathing pattern abnormalities, mostly described by an Apnea/Hypopnoea Index (AHI), only weakly correlate with quantified measures of sleepiness, such as the Epworth Sleepiness Scale (ESS). This probably means that interindividual sensitivity, with some individuals coping better with sleep fragmentation than others, does compromise the relationship between the AHI and daytime sleepiness scores. In addition, epidemiological studies show a broad range of sleepiness in the general population. Obviously, epidemiological studies investigating the prevalence of OSAHS are all biased by the lack of a uniform definition. The prevalence of an AHI of >5 events · h-1 in a general population (without taking into account symptoms of sleepiness) has previously been estimated to be 24% in a male population. When symptoms of sleepiness were also taken into account, the prevalence decreased to 4% in males and 2% in females.