Sleep is generally beneficial and restorative to a person. Therefore, it exerts a great influence on a person's quality of life. The human sleep/wake cycle generally conforms to a circadian rhythm that is regulated by a biological clock. Regular periods of sleep enable the body and mind to rejuvenate and rebuild. The body may perform various tasks during sleep, such as organizing long term memory, integrating new information, and renewing tissue and other body structures.
Lack of sleep and/or decreased sleep quality may have a number of causal factors including, e.g., respiratory disturbances, nerve or muscle disorders, and emotional conditions, such as depression and anxiety. Chronic long-term sleep-related disorders such as chronic insomnia, sleep-disordered breathing, and sleep movement disorders may significantly affect a patient's sleep quality and quality of life.
Sleep apnea, for example, is a fairly common breathing disorder characterized by periods of interrupted breathing experienced during sleep. Sleep apnea is typically classified based on its etiology. One type of sleep apnea, denoted as obstructive sleep apnea, occurs when the patient's airway is obstructed by the collapse of soft tissue in the rear of the throat. Central sleep apnea is caused by a derangement of the central nervous system control of respiration. The patient ceases to breathe when control signals from the brain to the respiratory muscles are absent or interrupted. Mixed apnea is a combination of the central and obstructive sleep apnea types. Regardless of the type of apnea people experiencing an apnea event stop breathing for a period of time. The cessation of breathing may occur repeatedly during sleep, sometimes hundreds of times a night and occasionally for a minute or longer.
In addition to apnea, other types of disordered breathing have been identified, including, for example, hypopnea (shallow breathing), dyspnea (labored breathing), hyperpnea (deep breathing), and tachypnea (rapid breathing). Combinations of the disordered respiratory events described above have also been observed. For example, Cheyne-Stokes respiration (CSR, which is sometimes referred to as periodic breathing) is associated with rhythmic increases and decreases in tidal volume caused by alternating periods of hyperpnea followed by apnea or hypopnea. The breathing interruptions of CSR may be associated with central apnea, or may be obstructive in nature. CSR is frequently observed in patients with congestive heart failure (CHF) and is associated with an increased risk of accelerated CHF progression.