Apnea is a pause in breathing for 20 seconds or more. Premature infants younger than 32 to 34 weeks' gestational age commonly have apneic spells, usually while sleeping. During an apneic spell, an infant's blood oxygen level can drop (oxygen desaturation, or "desat"), which is sometimes followed by a drop in heart rate (bradycardia).

The cause of apnea of prematurity is poorly understood. It is known to be related to the infant's immature neurological, muscular, and respiratory development.

After apnea spells have stopped for a week or more, they usually do not recur.