Although sleep-disordered breathing and cognitive impairment are common features of patients with myotonic dystrophy type 1 (DM1), little is known about their relationship. Forty-three adult DM1 patients underwent 2 sequential polysomnographic sessions and a neuropsychological test battery (intellectual functioning, attention, language, memory, and executive functions). Lower scores in attention, vigilance, and executive functioning tests were associated with higher number of apneic/hypopneic episodes per hour of sleep and longer total sleep time at an oxyhemoglobin saturation of less than 90%. Results suggest a potential role for nighttime breathing problems in the cognitive impairment often observed in DM1 patients.