Occupational, family, and social pressures have led to a society that sleeps less. In 1960, a survey by the American Cancer Society found that people slept, on average, 8 h per night. This figure has dropped today to 6.7 h, a decrease of 15% in less than one generation. This fall may be due to medical conditions, sleep disorders, or medications that inhibit sleep, as well as lifestyle disrupters such as jet lag, exam stress, or parenting infants. With the change toward a 24-h society, working outside of the 9-to-5 day has also become increasingly prevalent. Shift workers commonly obtain less sleep than permanent daytime workers as daytime sleep is sacrificed to accommodate social, family, and leisure activities. Sleep deprivation causes between $43 billion and $56 billion worth of accidents annually, as well as contributing to a substantial number of injuries and deaths worldwide. Humans are biologically programmed to sleep at night, and going without sleep has a range of deleterious consequences for cognitive performance, health, and well-being. This article will first outline the methodologies used to investigate partial sleep deprivation in humans and animals and discuss the neurobehavioral, physiological, and psychosocial consequences of reduced sleep. The article will then describe the function of sleep stages, again summarizing methodologies and reviewing the findings from rapid eye movement (REM) and slow-wave sleep (SWS) selective sleep stage deprivation studies in humans and animals.