The aim of this study was to document the clinical picture of excessive daytime sleepiness (EDS) and of other sleep disturbances, and to study the relationship of daytime sleepiness to anthropometric data, muscular impairment, and CTG trinucleotide repeat expansion in myotonic dystrophy type 1 (DM1). A total of 157 DM1 patients were surveyed using a modified version of the Sleep Questionnaire and Assessment of Wakefulness. Other measurements included muscular impairment rating and the size of the trinucleotide repeat. Factor analysis and reliability estimates were used to produce a daytime sleepiness scale with five items of the questionnaire. Thirty-eight healthy family members were studied as control subjects. It was found that EDS was present in 33.1% of DM1 patients. Severity of daytime sleepiness correlated with the degree of muscular impairment but not with age, gender, body mass index, age at onset of symptoms, duration of illness, and CTG repeat. DM1 patients reported a longer sleep period, a less restorative sleep, and more difficulty falling asleep, being alert in the morning and staying awake after meals than controls, but a similar incidence of narcolepsy auxiliary symptoms. Compared with DM1 patients without EDS, those with EDS reported greater hypnagogic hallucinations, and greater pain associated with nocturnal awakenings and in their legs upon morning awakenings. In sum, both DM1 patients with and without EDS exhibit characteristics of sleep duration and sleepiness comparable with those found in idiopathic hypersomnia. The severity of daytime sleepiness is weakly related to the extent of muscular impairment but not to CTG repeat.