Until 2010, modafinil, which is a wakefulness promoting agent, was approved in Europe for a wider spectrum of indications, such as narcolepsy, idiopathic hypersomnia, obstructive sleep apnoea and shift work sleep disorder. Currently, it is registered by the European Medicines Agency only for the treatment of narcolepsy, and is used as an off-label therapy in other sleep disorders. This paper presents the efficacy of modafinil in selected sleep disorders. Modafinil remains first-choice treatment for narcolepsy. It reduces the frequency of bouts of inadvertent sleep and nap episodes, the duration and intensity of daytime hypersomnolence, and also significantly improves the quality of life of patients. However, it is associated with only a slight improvement in cataplexy and other symptoms. In idiopathic hypersomnia, modafinil reduces the frequency of naps and unintentional sleep episodes, as well as subjective sleepiness measured with the Epworth Sleepiness Scale. Furthermore, the drug is used to treat hypersomnia from obstructive sleep apnoea in the case of lack of improvement despite optimal positive airway pressure therapy. Modafinil is also approved by the U.S. Food and Drug Administration for the treatment of shift work sleep disorder. The drug has been shown to reduce the level of somnolence, but it has not been found to reduce unintentional sleep episodes, reported mistakes or accidents at work. Given the strong negative impact of hypersomnolence on performance at work and school, the risk of accidents and the quality of life, the risk-benefit assessment of modafinil often justifies its use in the treatment of hypersomnolence also outside the approved indications.