Apnea means suspension of breathing, which is either voluntary or involuntary, that is caused by drugs or some external reason like strangulation, or trauma. During apnea the respiratory muscles do not function, and the volume of the lungs remains unchanged. As body cannot store oxygen, a minute of apnea causes much decline of oxygen in the blood. Apnea that lasts for about three minutes can damage the brain. If the stage continues it can result in death. Only in special conditions like apneic oxygenation or hypothermia can life be sustained longer in the event of an apnea. Without proper training more than two minutes of apnea causes carbon dioxide in the blood to rise and PH level to drop, causing damage to the brain. Apneist is the name given to a person who can stop breathing for longer periods, and expert divers who practice holding breath can remain in the state of apnea up to nine minutes under water. Mammals under water can hold their breath a little longer owing to a survival phenomenon known as mammalian diving reflex. Voluntary hyperventilation causes the carbon dioxide in the blood to rise, due to which the respiratory centre is not stimulated in time and breathlessness is not felt. Divers use hyperventilation to hold their breath for longer time in water owing to this factor. It is however dangerous because before they breathe the blood has lost all oxygen, causing unconsciousness. Blackout under water can cause serious accidents and drowning. Apneic oxygenation is exchange of gases between lungs and blood that continues even during the state of apnea. It is a phenomenon in which the level of oxygen absorbed is more than carbon dioxide released, and the lungs collapse. If pure oxygen is breathed in before apnea the individual can survive up to an hour in that state and the technique can be used in case of emergencies. Respiratory acidosis is a condition that can occur due to accumulation of carbon dioxide in the body tissues in course of apneic oxygenation. Brain death is a condition in which an individual is unable to breathe without life support systems. When there is cessation of breath during sleep the condition is known as sleep apnea. It occurs because the level of oxygen in blood decreases on account of some blocks in the air passage through mouth or nose, due to the tongue or uvula, structure of the jaw, swelling or relaxing of certain muscles etc. Pause that lasts up to 10 second can be considered apnea, if it repeats through out the night. Patients of sleep apnea suffer from daytime sleepiness and confusions that result from lack of proper rest. Their work and relations suffer due the irritability, lethargy and lack of concentration that set in as a result of prolonged periods of sleeplessness. Snoring is a common feature among the patients of this ailment and during the apnea there is a pause in snoring, which resumes once the individual gasps for breath and starts respiration. Obstructive sleep apnea occurs during the night in the form of cessation of breath Central sleep apnea occurs when the nerves are not strong enough to control the respiratory system due to use of central respiratory depressants like alcohol and other sedatives. It can be dangerous for someone who already suffers from sleep apnea to use a central respiratory depressant. Central sleep apnea is also seen in infants, particularly those who are born prematurely. Complex sleep apnea is the combination of both obstructive and central sleep apnea.