While sleeping, a normal individual is "at rest" as much as cardiovascular workload is worried. Breathing is regular in the healthy person during sleeping, and oxygen levels and carbon dioxide levels in the our blood stay fairly constant. The respiratory drive can be so strong that even conscious efforts to carry one's breath do not overcome it. Any sudden drop in oxygen or more than carbon dioxide (even if tiny) strongly encourages the brain's respiratory centers to breathe. My websites: How Snooze Apnea Affects Your Wellbeing