There are two types of sleep apnea: central apnea and obstructive apnea. In central apnea, the brain does not send the proper signals to breathe during sleep; this is a rare condition but can be treated. In obstructive apnea, which is by far the most common type, the patient tries to breathe but cannot because of tissue obstruction in the upper airway which is more pronounced when the patient lies down, or from muscle relaxation during deep sleep. This obstruction can be from excess tissue, and also from disproportionate anatomy in the upper airway. Snoring is caused by vibration of excess soft tissue in the upper airway, usually the soft palate and uvula. Since both snoring and obstructive sleep apnea can be caused by excess tissue, many patients have both, but again they are not always related. Both sleep apnea and snoring can be caused or worsened by sleep position; typically both are worse while sleeping on the back.

Obstructive sleep apnea is a uniquely human problem and can be considered a price we pay for our ability to talk. We use our throat in at least three different ways. We use it to forms words when we speak, to propel food when we swallow, and to serve as a passageway for air when we breathe. We are stuck with a single tube that must be flexible and collapsible so we can talk and swallow, but must stiffen up to resist collapse when we suck air into our lungs. The solution to this design problem is a complex group of muscles that change the shape of our throat when we talk and swallow, but also stiffen and dilate the passageway when we breathe in. These muscles work well when we are awake, but like all muscles they relax – become less active – when we are asleep. If our airway is abnormal in its size or shape or "stiffness", for example if it is too small because of excess tissue in or around it, then the muscles responsible for holding it open during sleep are unable to do their job. The airway collapses so no air or not enough air gets to the lungs.

So OSA is caused by conditions that narrow this passageway- the upper airway- or make it more collapsible. Chief among these is obesity, especially obesity with a large neck, although other conditions such as having "kissing" tonsils or a relatively small jaw (this gives one a relatively large tongue) also can promote upper airway collapse. Certain diseases such as hypothyroidism (low thyroid hormone levels) are also associated with OSA. The effect of gravity on the tongue and other structures surrounding the upper airway can narrow it when sleeping on your back. Drinking alcohol near bedtime can make the airway more collapsible.