A system can perform the following procedure as part of a plan to treat sleep apnea, according to an embodiment. The system monitors a degree to which an airway of a subject is open, and monitors a condition that is related to the degree to which the airway is open. The system also correlates the condition to the degree to which the airway is open, and generates a data structure that represents the correlation between the condition and the degree to which the airway is open. This system can allow a sleep-apnea-treatment system to operate in a closed-loop mode, i.e., to monitor a level of sleep apnea in a subject by monitoring the correlated condition, and to treat the subject in a manner that the treatment system deems appropriate for the level of sleep apnea that the treatment system determines that the subject is experiencing.