Historically, sleep deprivation was not viewed as a significant problem, and the consequences of nighttime sleep loss were thought to be limited to daytime sleepiness. We now know, however, that sleep loss is associated with multiple detrimental health consequences, including obesity, diabetes, cardiovascular disease, and hypertension. In the extreme, studies of laboratory animals indicate that prolonged sleep deprivation results in death. Sleep loss is an increasing problem in modern society for many reasons, including lifestyle choices. This entry focuses on the detrimental consequences of sleep loss on immunity. For the purpose of this entry, short sleep is defined as a sleep duration of less than 7 hours a night; the biological sleep need is generally considered to be 7 to 8 hours a night.