Myotonic dystrophy type I (DM1) is one of the most variable of all human disorders, with virtually all body systems affected in some way, and age at disease onset varies from fetal life to old age. The character of DM1-related excessive daytime sleepiness (EDS) is that of a persistent sleepiness unaffected by naps, the latter being long, unrefreshing and without any associated dream. This chapter presents the scores of DM1 patients with and without EDS on the eight Short-Form 36-item Health Survey subscales, a frequently used generic health-related quality of life (HRQoL) questionnaire. It describes the personality and cognitive characteristics of 200 DM1 patients with and without EDS. The chapter discusses the habitual sleep-wake schedule and sleep-related complaints of 200 DM1 patients with and without EDS. Sleep-disordered breathing (SDB), hypercapnia, and nocturnal desaturation are all frequent in DM1. Psychostimulant drugs are increasingly used to treat EDS in DM1.