There are two main types of sleep apnea: central sleep apnea, a relatively uncommon form which occurs when the brain fails to send the signal to the muscles to take a breath, and so there are periods in which there is just no muscular effort to breathe; and obstructive sleep apnea (OSA), a much more common form, where the brain sends signals to the muscles and the muscles do make an effort to take a breath, but they are unable to comply effectively because an obstructed airway prevents an adequate flow of air. Mixed sleep apnea, a mixture of the two, is also possible but extremely uncommon.
Sleep hypopnea is similar in many respects to sleep apnea, with similar symptoms and long-term effects, although it is less common. It is caused by excessively shallow breathing or an abnormally low respiratory rate, rather than by complete breathing pauses due to obstructions or other causes. This typically leads to loud, heavy snoring, interrupted several times an hour by choking sounds or snorts followed by a silence of up to 20 seconds as insufficient air flows into the lungs. Sleep hypopnea and the various kinds of sleep apnea mentioned above are often lumped together under the heading sleep-disordered breathing. Upper airway resistance syndrome is a similar, but milder, form of sleep-disordered breathing.
In the most common case of obstructive sleep apnea, breathing is disrupted by a physical block to the airflow caused by a collapse of the in the throat and respiratory tract (the same tissues as produce the sound of snoring, which almost always accompanies sleep apnea). During these pauses in breathing (or apneas), the sleeper effectively starts to suffocate (in addition to experiencing a sharp loss in blood oxygen levels and increased carbon dioxide levels), and the brain sends an immediate emergency signal to the body to wake up. When the sleeper wakes and takes a deep breath, the brain is replenished and the person is able to go back to sleep.
Each apnea episode may last from just a few seconds to minutes (at least 10 seconds is the usual benchmark, whether referring to obstructive or central sleep apnea), and they may occur anywhere from 5 to 30 or more times an hour. Because of the complete loss of muscle tone associated with REM sleep, that is the most common time to experience apneas, and they are relatively rare during non-REM sleep.
The frequent partial awakenings that are caused by sleep apnea (often referred to as micro-awakenings) lead, over time, to chronic sleep deprivation and excessive daytime sleepiness. Sleep apnea can disturb sleep to such an extent that, even after a full night in bed, the sleeper does not feel rested due to the compromised quality of the sleep. In addition to excessive daytime sleepiness, the constant sleep interruptions may lead to loss of energy, headaches, forgetfulness, and, in the longer term, to high blood pressure, heart attack, stroke, depression, or other mood disorders. The lowered blood oxygen level during sleep apnea can also disrupt the stages of sleep in a person's sleep cycles.
Men are more likely to suffer from sleep apnea than women and children, and the risk increases with age, body weight and smoking habits. In particular, sleep apnea is highly correlated with obesity, mainly due to excess fat on the sides of the neck that can cause obstructions in the airway. Several other conditions also increase the risk of sleep apnea, including menopause, asthma, epilepsy, Down syndrome, etc. In children, it may result from enlarged tonsils or adenoids.
Often, a sufferer from sleep apnea is completely unaware of the condition, which may therefore continue for years without identification. Even then it may only be identified by someone else witnessing the episodes or complaining about the loud snoring that usually accompanies it, or merely as a consequence of the ongoing daytime sleepiness, fatigue and irritability that result from it.
It has become apparent in recent years that those with sleep apnea are more vulnerable to a variety of parasomnias, such as sleep-walking, sleep-sex, sleep-eating, confusional arousals, night terrors, etc, as well as other sleep disorders like periodic limb movement disorder, restless legs syndrome, etc.
Common treatments include lifestyle changes (such as avoiding alcohol and other muscle relaxants, losing weight and quitting smoking), sleeping on a 30 degree angle or on one’s side, various kinds of oral/dental appliances or masks (particularly continuous positive airway pressure, or CPAP, masks, which remains the leading therapy for severe sleep apnea), or even surgical procedures to increase the size of the airway.