The epidemiological study of hypersomnia symptoms is still in its infancy; most epidemiological surveys on this topic were published in the last decade. More than two dozen representative community studies can be found. These studies assessed two aspects of hypersomnia: excessive quantity of sleep and sleep propensity during wakefulness excessive daytime sleepiness. The prevalence of excessive quantity of sleep when referring to the subjective evaluation of sleep duration is around 4% of the population. Excessive daytime sleepiness has been mostly investigated in terms of frequency or severity; duration of the symptom has rarely been investigated. Excessive daytime sleepiness occurring at least 3 days per week has been reported in between 4% and 20.6% of the population, while severe excessive daytime sleepiness was reported at 5%. In most studies, men and women are equally affected. In the International Classification of Sleep Disorders, hypersomnia symptoms are the essential feature of three disorders: insufficient sleep syndrome, hypersomnia (idiopathic, recurrent or posttraumatic) and narcolepsy. Insufficient sleep syndrome and hypersomnia diagnoses are poorly documented. The co-occurrence of insufficient sleep and excessive daytime sleepiness has been explored in some studies and prevalence has been found in around 8% of the general population. However, these subjects often have other conditions such as insomnia, depression or sleep apnea. Therefore, the prevalence of insufficient sleep syndrome is more likely to be between 1% and 4% of the population. Idiopathic hypersomnia would be rare in the general population with prevalence, around 0.3%. Narcolepsy has been more extensively studied, with a prevalence around 0.045% in the general population. Genetic epidemiological studies of narcolepsy have shown that between 1.5% and 20.8% of narcoleptic individuals have at least one family member with the disease. The large variation is mostly due to the method used to collect the information on the family members; systematic investigation of all family members provided higher results. There is still a lot to be done in the epidemiological field of hypersomnia. Inconsistencies in its definition and measurement limit the generalization of the results. The use of a single question fails to capture the complexity of the symptom. The natural evolution of hypersomnia remains to be documented.