It's not clearly understood what causes these spells. Some researchers think the spells are related to centers inside the brain that regulate breathing and that may not be fully developed. Touching your baby gently or rocking the incubator slightly will almost always get the baby to start breathing again, which increases the heart rate.
Central apnea occurs when the child seems to 'forget' to breathe.
Obstructive apnea occurs when the child makes some effort to breathe, but the voice box or upper airway is closed. The voice box can close in response to food refluxing up from the stomach or just because the infant is small and cannot keep the airway open. Prematurely born babies often have a combination of these two types of apnea. However, prematurely born babies are more likely to get SIDS just because they are prematurely born. No one knows exactly why this is the case.