STUDY OBJECTIVES: Sleep studies at Duke University Medical Center were retrospectively reviewed to investigate causes of excessive daytime sleepiness (EDS) in our myotonic dystrophy1 (DM1) patient population, identified by an abnormal CTG expansion on chromosome 19. Excessive daytime sleepiness, an accentuated desire for sleep or the occurrence of sleep episodes that interfere with normal wakefulness, is common in patients with DM1. Sleep abnormalities, such as central and obstructive sleep apnea, have been extensively reported; however, many DM1 patients suffer from EDS in the absence of any identified respiratory dysrhythmia. DESIGN: Nineteen DM1 patients, with the clinical diagnosis and genetic confirmation in the proband or a related family member, had a sleep evaluation. Polysomnogram (PSG) and mean sleep latency test (MSLT) results were retrospectively reviewed. SETTING: N/A. PARTICIPANTS: N/A. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: Most DM1 patients demonstrated grade 3 (distal weakness) or grade 4 (mild to moderate proximal weakness) on the myotonic dystrophy impairment rating scale. All patents had a PSG, with 13 patients having an MSLT the following day. Clinically significant respiratory abnormalities on PSG, defined in this study as a respiratory disturbance index > 15 and/or upper airway resistance syndrome, could not explain the EDS observed in 14 of 19 patients. Decreased mean sleep latency was observed in 12 of 13 patients evaluated by MSLT, while sleep onset REM sleep was seen in 8 of 13 patients. Pathologic REM onset (2 or more SOREMs on MSLT) was seen in 5 of 13 patients, with 3 of those 5 patients having a PSG with RDI < or = 5. CONCLUSIONS: Most DM1 patients did not have significant respiratory abnormalities on PSG to explain the manifested EDS. Objective sleepiness is common in DM1, and pathologic REM pressure can be commonly observed. These observations imply an intrinsic hypersomnolence sometimes accompanied by abnormal REM pressure may be an integral part of EDS in DM1 patients.