Excessive daytime sleepiness (EDS) is a prevalent complaint among patients in psychiatric and medical care. Patients with EDS have often been misdiagnosed with depression due to their complaints of lack of energy and poor concentration. Also, they have even been diagnosed erroneously with a psychotic disorder in case of coexistence with hypnagogic hallucination. EDS can seriously affect the person's quality of life by causing decreased academic achievement or work performance, low self esteem, and social withdrawal. EDS is also frequently associated with various medical and psychiatric conditions, and often fatal traffic or on-the-job accidents. The causes of EDS range from insufficient sleep to central nervous system-originated hypersomnia. The conditions that can lead to EDS include circadian rhythm disorders, primary disorders of alertness such as narcolepsy, sleep-related breathing disorders such as obstructive sleep apnea syndrome, sleep-related movement disorders such as periodic limb movement disorder and restless legs syndrome, chronic medical conditions such as cancer, and medications causing sleepiness. Treatment options should be tailored according to the underlying condition and include sufficient sleep time, light therapy, sleep scheduling, wakefulness-promoting medications, or mechanical airway managements such as nasal continuous positive airway pressure (CPAP).