This is an unnatural breathing rhythm in which the sufferer alternates between panting and temporary cessation of breathing. It often occurs during sleep, and can be quite alarming for a companion if the pause is more than about 30 seconds (it may also cause the sleeper to wake up). The cause is related to the two mechanisms by which the body regulates involuntary breathing.

The principal mechanism is a response to carbon dioxide (CO2) in the lungs: when a certain concentration, is reached, there is a reflex to exhale*. The lungs then refill with fresh, oxygenated air from outside. This may be why some climbers report that smoking helps them to breathe better at altitude, as the additional CO2 from burning tobacco may stimulate the exhalation reflex. Contrary to popular supposition, there is no such reflex for a low concentration of oxygen in the lungs, which is why it can be easier to avoid breathing (such as when one is close to drowning) when the lungs are empty than when they are full. The second regulatory mechanism concerns the proportion of oxygen in the blood, which is measured in the brain stem, presumably a very ancient evolutionary development. Too low a value causes an involuntary inhalation (as in a yawn). It is apparently the alternation of these two mechanisms that causes Cheyne-Stokes breathing.

Cheyne-Stokes breathing, solely as a direct result of increased altitude, is common, and is generally not a cause for concern, unless it continues after acclimatization would have been expected, or is accompanied by unexpected symptoms.