Figure 3: Neurophysiologic features of the central respiratory system favour the appearance of instabilities during sleep. A: Short-term potentiation prevents apneas after augmented breaths during wakefulness. This effect is blunted during sleep. B: The lung inflation reflex may elicit apneas after sighs during sleep and may trigger periodic breathing. C: If the PCO 2 falls below the apnea threshold, which is a few Torr below eucapnic values, the respiratory rhythm ceases. D: Due to the inherent control system inertia, the respiratory drive has to exceed the pre-apneic values markedly in order to re-establish the respiratory rhythm.