Excessive daytime sleepiness (EDS) is a public health issue. However, it remains largely undervalued, scarcely diagnosed, and poorly supported. Variations in the definition of EDS and limitations in clinical assessment lead to difficulties in its epidemiological study, but the relevance of this symptom from a socioeconomic perspective is inarguable. EDS might be a consequence of several behavioural issues leading to insufficient or disrupted sleep, as well as a consequence of sleep disorders including sleep apnoea syndrome, circadian disorders, central hypersomnolence disorders (narcolepsy and idiopathic hypersomnia), other medical or psychiatric conditions, or medications. Furthermore, EDS can have implications for health as it is thought to act as a risk factor for other conditions, such as cardiovascular and neurodegenerative disorders. Because of the heterogeneous causes of EDS and the complexity of its pathophysiology, management will largely depend on the cause, with the final aim of making treatment specific to the individual using precision medicine and personalised medicine.