COPD Clinical Perspectives 118response to mechanical, cortical, and metabolic stimuli [ , ]. The ventilatory and arousal responses to hypercapnia are much more robust than for hypoxia, with only slight changes in PaCO causing recognizable alterations of minute ventilation [ ]. However, under normal conditions in sleep and especially during periods of REM, the responses to hypoxaemia and hypercapnia are blunted in comparison to wakefulness. The diminished ventilatory responses to hypercapnia and hypoxia are more profound during REM compared with NREM sleep [ , ]. These attenuated feedback mechanisms stress the respiratory system which in individuals with COPD is already deranged and dysfunctional.