Depends on the patient and the cause. Some require them 24/7. In the case of polio, the nerves controlling the diaphragm are sometimes affected. This can be

Depends on the patient and the cause. Some require them 24/7. In the case of polio, the nerves controlling the diaphragm are sometimes affected. This can be partial, with a reduced but present breathing, or completely absent. In either case, but especially when only partial, people can often adapt when conscious and upright. Other muscles may not be affected, and they can use non-diaphragm muscles to expand/contract the rib cage. In mild cases this is just an assist and provides the extra ventilation for strenuous activity. In serious cases it's required to keep them conscious. While such adaption becomes an unconscious reflex, it's still largely suppressed when asleep. There are some parallels to how people with obstructive sleep apnea adapt. They may be able to breathe (e.g. through their mouth) during the day but at night, the instinctive breathing process isn't working quite right. The result is usually some kind of sleep apnea. Waking up feeling suffocated and having to will yourself to breathe. In the extreme, if that reflex doesn't kick in either, it could cause a hypoxic event, that inhibits breathing even more, and that's potentially fatal.