Hypersomnolence has been difficult to describe for centuries. When patients present with symptoms of excessive daytime sleepiness, often this is due to one or a combination of five major causes: (1) an insufficient quantity of sleep; (2) poor quality of sleep (e.g., related to sleep-disordered breathing, adverse environment—e.g., excess noise); (3) a pathologic disorder of sleepiness during the day, often referred to as a hypersomnolence disorder; (4) a medical or mental health disorder, or medication issue; and (5) a circadian rhythm disorder, such as shift work, or travel to different time zones. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), characterized hypersomnolence disorder as a condition in which the person reports feeling overly sleepy despite having slept for a [Page 1714]minimum of ...