OBJECTIVES In patients who exhibit myotonic dystrophy type 1 (DM1), sleep disorders and breathing impairments are common; however, in those with DM type 2 (DM2), limited studies on polysomnography (PSG) and none on phrenic compound motor action potential (CMAP) have been performed, which is the aim of this study. MATERIALS AND METHODS Sixteen patients with DM2 were questioned about respiratory symptoms. They underwent PSG with morning arterial gas analyses (AGA). Respiratory functions and phrenic CMAPs were studied. The data were compared to those of 16 healthy controls and 25 patients with DM1. RESULTS Daytime tiredness is the most common symptom, but orthopnea was reported in 13% of patients with DM2. A detailed sleep architecture analysis revealed a significantly greater proportion of time in stage 3 and REM sleep, and a shorter time in stage 2 in the DM2 than in controls. Lower respiratory volumes and pressures, abnormalities in AGA, night oxygen desaturation and higher EtCO2 are present in DM2, but are less pronounced than in the DM1 population. Small CMAP amplitudes were presented in 12% of patients with DM2, correlating with smaller respiratory functions and poorer sleep quality. AHI was abnormal in 38% of DM2, mainly due to obstructive apneas. PSG did not reveal hypoventilation. CONCLUSIONS Diaphragm weakness and sleep apneas might be present in patients with DM2; therefore, we suggest regular questioning about symptoms of respiratory insufficiency and monitoring of phrenic CMAP. PSG should be recorded, when patients have suggestive symptoms, abnormalities in AGA or higher BMI.