Figure 1: Irregular, rapid and shallow breathing during phasic rapid eye movement (REM) sleep. Decrease in ventilation during phasic REM sleep episodes: a sustained reduction in flow and thoracic and abdominal movements occurs concurrently with rapid eye movements. This occurs to various degrees in normal subjects and is aggravated or revealing in patients with respiratory failure. SpO2: Arterial oxygen saturation measured by pulse oximetry; Tho: Thoracic movements; Abd: Abdominal movements; Flow: Flow signal; PTT: Pulse transit time; ABP: Arterial blood pressure; EM: Eye movements. Reproduced with permission of the European Respiratory Society©.Eur Respir Mon November 2008 41:350-366.