Hypersomnia is a neurological disorder of excessive time spent sleeping or excessive sleepiness. It can have many possible causes (such as seasonal affective disorder) and can cause distress and problems with functioning. In the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), hypersomnolence, of which there are several subtypes, appears under sleep-wake disorders.

Hypersomnia is a pathological state characterized by a lack of alertness during the waking episodes of the day. It is not to be confused with fatigue, which is a normal physiological state. Daytime sleepiness appears most commonly during situations where little interaction is needed.

Since hypersomnia impairs patients' attention levels (wakefulness), quality of life may be impacted as well. This is especially true for people whose jobs request high levels of attention, such as in the healthcare field.