Fig 7. Ventilatory response in the transition from wakefulness to sleep while breathing 15% O2. Awake, alert ventilation during long term hypoxia while breathing 15% O2 is stable. However, sleep is destabilizing. The drop in oxygen saturation during sleep to an average of 89.4 % from 93% awake raises the controller CO2 gain from 2.7 to 3.5 liters/minute/mm Hg of PCO2 in sleep, enough to cause the instability. Even without HVD the ventilatory response is unstable. The alertness factor awake is 0 liters/min, the alert state, and asleep it is -11.5 liters/min.