Sleep deprivation, also known as sleep insufficiency or sleeplessness, is the condition of not having adequate duration or quality of sleep to support decent alertness, performance, and health. It can be either chronic or acute and may vary widely in severity. This means it can happen over both short and long periods of time. Sleep is important because adequate sleep, or restful sleep, is essential for maintaining your overall health, brain performance, emotional regulation, and metabolic balance. Persistent sleep insufficiency can contribute to cognitive decline, emotional instability, and biological wear that has effects similar to accelerated aging. Scientific research demonstrates overwhelming evidence that inadequate sleep produces chronic consequences for overall health, ranging from attentional lapses to long-term neurodegenerative changes.

The human body and most living organisms depend on sleep for neural recovery. All known animals sleep or exhibit some form of sleep behavior, and the importance of sleep is self-evident for humans, as nearly a third of a person's life is spent sleeping. Sleep deprivation is common as it affects about one-third of the population.

The National Sleep Foundation recommends that adults aim for 7 hours of sleep per night. Children and teenagers require even more sleep, ranging from 8–16 hours per night. For healthy young individuals with normal sleep, the appropriate sleep duration for school-aged children is between 9 and 11 hours. Acute sleep deprivation occurs when a person sleeps less than usual or does not sleep at all for a short period, typically lasting one to two days. However, if the sleepless pattern persists without external factors, it may lead to chronic sleep issues. Chronic sleep deprivation occurs when a person routinely sleeps less than the amount required for proper functioning. The amount of sleep needed can depend on sleep quality, age, pregnancy, and level of sleep deprivation. Sleep deprivation is linked to various adverse health outcomes, including cognitive impairments, mood disturbances, and increased risk for chronic conditions. A meta-analysis published in Sleep Medicine Reviews indicates that individuals who experience chronic sleep deprivation are at a higher risk for developing conditions such as obesity, diabetes, and cardiovascular diseases.

Insufficient sleep has been linked to weight gain, high blood pressure, diabetes, depression, heart disease, and strokes. Sleep deprivation can also lead to high anxiety, irritability, erratic behavior, poor cognitive functioning and performance, and psychotic episodes. A chronic sleep-restricted state adversely affects the brain and cognitive function. However, in a subset of cases, sleep deprivation can paradoxically lead to increased energy and alertness; although its long-term consequences have never been evaluated, sleep deprivation has even been used as a treatment for depression.

To date, most sleep deprivation studies have focused on acute sleep deprivation, suggesting that acute sleep deprivation can cause significant damage to cognitive, emotional, and physical functions and brain mechanisms. Few studies have compared the effects of acute total sleep deprivation and chronic partial sleep restriction. A complete absence of sleep over a long period is not frequent in humans (unless they have fatal insomnia or specific issues caused by surgery); it appears that brief microsleeps cannot be avoided. Long-term total sleep deprivation has caused death in lab animals.

Reviews differentiate between having no sleep over a short-term period, such as one night ('sleep deprivation'), and having less than required sleep over a longer period ('sleep restriction'). Sleep deprivation was seen as more impactful in the short term, but sleep restriction had similar effects over a longer period. A 2022 study found that in most cases the changes induced by chronic or acute sleep loss waxed or waned across the waking day.

Sleep debt refers to a build up of lost optimum sleep. Sleep deprivation is known to be cumulative. This means that the fatigue and sleep one lost as a result of, for example, staying awake all night, would be carried over to the following day. Not getting enough sleep for a couple of days cumulatively builds up a deficiency and causes symptoms of sleep deprivation to appear. A well-rested and healthy individual will generally spend less time in the REM stage of sleep. Studies have shown an inverse relationship between time spent in the REM stage of sleep and subsequent wakefulness during waking hours. Short-term insomnia can be induced by stress or when the body experiences changes in environment and regimen.

Insomnia is a sleep disorder where people have difficulty falling asleep, or staying asleep for as long as desired. Insomnia may be a factor in causing sleep deprivation. There are three different types of insomnia; Transient insomnia, which means short-term sleep problems that last less than three weeks. Acute insomnia; which last for 1-3 weeks and Chronic insomnia; which last for at least 3 months and happens at least 3 nights per week. Insomnia can be caused by stress, anxiety, depression, medications, poor sleeping habits, and genetics. You can treat it with lifestyle changes like establishing a regular sleep schedule or changing your bedtime routine. There are different medications like sedatives and antidepressants, and sleep aids or Cognitive behavioral therapy (CBT) for insomnia.