CPAP dial-down method for measuring phenotypic traits. (i) Veupnea, ventilation at optimal CPAP with no evidence of snoring or flow limited breathing; (ii) passive V0, ventilation at CPAP = 0 cm H2O with completely relaxed pharyngeal muscles; (iii) Varousal, the ventilation just prior to a respiratory-induced arousal; (iv) active V0, ventilation at CPAP = 0 cm H2O with maximally activated pharyngeal muscles; (v) loop gain, the ratio of ventilatory overshoot (response) above Veupnea when returning to optimal CPAP pressure from a period of sub-optimal CPAP with reduced ventilation (disturbance).