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Sleep deprivation, a sleep disorder characterized by having too little sleep, can be either chronic or acute. Long-term sleep deprivation causes death in lab animals. A chronic sleep-restricted state can cause fatigue, daytime sleepiness, clumsiness and weight loss or weight gain.

Approximately 50 to 70 million Americans are reportedly affected by sleep deprivation or other sleep related disorders.

Complete absence of sleep over long periods is impossible for humans to achieve (unless they suffer from fatal familial insomnia); brief microsleeps cannot be avoided.

Sleep deprivation can adversely affect the brain and cognitive function. A 2000 study, by the UCSD School of Medicine and the Veterans Affairs Healthcare System in San Diego, used functional magnetic resonance imaging technology to monitor activity in the brains of sleep-deprived subjects performing simple verbal learning tasks. The study showed that regions of the brain's prefrontal cortex displayed more activity in sleepier subjects. Depending on the task at hand, the brain would sometimes attempt to compensate for the adverse effects caused by lack of sleep.

The temporal lobe, which is a brain region involved in language processing, was activated during verbal learning in rested subjects but not in sleep deprived subjects. The parietal lobe, not activated in rested subjects during the verbal exercise, was more active when the subjects were deprived of sleep. Although memory performance was less efficient with sleep deprivation, greater activity in the parietal region was associated with better memory.