Systematic trends in the length of apneas were investigated in 8 sleep apnea patients, all of whom had more than 200 apneas per night. Regression analysis performed on the length of apneas by thirds of the night revealed significant linear trends for apneas in sleep stage 2, and for apneas in all sleep stages pooled together. There were no significant trends in the index of apnea density. We suggest that the across-night lengthening of apneas reflects a progressive increase in the arousal threshold, either by a compensatory deepening of sleep or by progressive changes in respiratory chemoreceptor sensitivity. The lack of significant trends in rapid eye movement (REM) sleep supports the conclusion that apnea termination in REM sleep is mediated by a different mechanism than in non-REM sleep.