Excessive daytime sleepiness (EDS) is defined as feeling sleepy or having difficulty staying awake or alert during the day combined with an extreme desire to sleep that occurs nearly every day for at least three months. Excessive daytime sleepiness may be caused by a sleep disorder, such as insomnia, obstructive sleep apnea, or narcolepsy; restless leg syndrome; periodic limb movement disorder; circadian rhythm disorders; mental health disorders, such as depression, anxiety, or schizophrenia; Parkinson’s disease; cancer; chronic pain; or other conditions, such as epilepsy; obesity; substance abuse; multiple sclerosis; voluntary or chronic sleep deprivation; an inherited genetic predisposition; smoking; lack of exercise; or the use of medications, such as melatonin and sleeping pills. Excessive daytime sleepiness can be a dangerous condition that has contributed to serious accidents and major catastrophes.