Brain cells need constant oxygen to live. When sleep apnea is present, the patient can miss one or more cycles of breathing. If the pause in breathing is long enough, the percentage of oxygen in the circulation will drop to a lower than normal level (hypoxemia) and the concentration of carbon dioxide will build to a higher than normal
level (hypercapnia). At this point when blood oxygen goes low enough for an extended period of time, the patient can actually stop breathing all together.
In most cases this is rare and the brain wakes the body. Having such a low level of oxygen in the brain can lead to seizures or cause brain damage.