Depending on the type of apnea (obstructive, central, or complex (a mix of the two), the body with those who have CA (central apnea) do not react to rising levels of CO2 in their blood...carbon dioxide, which is one of several waste products of living cells. We don't breathe when our oxygen gets too low in our blood. We breathe when our blood levels of CO2 rises to a point where our sensors make the diaphragm spasm, a slow spasm, and this induces an exchange of gases in the lungs. Problem solved!
During waking moments, we can still, some of us, experience the same phenomenon where we stop breathing because our O2 saturation is good, and then we don't breathe until the body senses a dangerous or uncomfortable rise in CO2 and the slow spasm happens, except it's a gasp, maybe because we're startled by the sudden desire to breathe, or due to its urgency. Or, we are breathing normally, but shallowly, maybe concentrating on something, and suddenly our body objects to the poor exchange rate, we still fully conscious and pondering something, and we get that urgent gasp as if we emerge from a long lap under the water in a pool.