We propose the use of proportional assist ventilation (PAV) as a method to quantitate ventilatory stability. The PAV amplification factor required to produce periodic breathing is the reciprocal of overall loop gain. Using this approach in 12 normal subjects we found that, after stabilization of upper airway with continuous positive airway pressure (CPAP) (5.2 +/- 2.0 cm H2O), overall loop gain was very low (< 0.3) indicating that sleep. per sc, is not destabilizing under normal conditions.