Excessive daytime sleepiness (EDS) is a prevalent complaint among patients in psychiatric care. Patients with conditions of EDS have often been misdiagnosed with depression due to their complaints of lack of energy, poor concentration, memory disturbance, and a reduced interest in life. Impaired alertness associated with EDS can be detrimental to a person's quality of life by causing decreased work performance, self-consciousness, low self esteem, and social isolation. Excessive sleepiness is also associated with various health problems, comorbid medical and psychiatric conditions, and fatal accidents occurring after the driver has fallen asleep at the wheel. Contributing factors leading to EDS range from insufficient sleep hours to central nervous system-mediated debilitating hypersomnolence. Circadian rhythm disorders, sleep disorders such as obstructive sleep apnea and narcolepsy, and medications that cause sleepiness may also contribute to symptoms of EDS. Recognition of the symptoms of sleep deprivation is essential, as many such patients do not have a clear awareness of their own sleepiness. Treatment options, depending upon the condition, include light therapy or appropriate airway management techniques such as nasal continuous positive airway pressure (CPAP). Occasionally, wakefulness-promoting medications are necessary, particularly in patients with narcolepsy. In this expert roundtable supplement, Stephen P. Duntley, MD, reviews the definition and prevalence of EDS and discusses the contributing factors and consequences of daytime sleepiness. Next, Richard K. Bogan, MD, FCCP, gives an overview of the differential diagnosis of EDS and the assessment tools available for identifying sleepiness in symptomatic patients. Finally, Mary B. O'Malley, MD, PhD, reviews treatment of EDS, including counseling on sleep hygiene and duration of sleep, mechanical treatments, bright-light therapy, and wake-promoting medications.