Sleep deprivation is a catch-all term for when the body doesn’t get sufficient sleep. Sleep deprivation may be brought about by a number of causes, from physical illness, to psychiatric imbalance, to torture, and can have a wide range of consequences. At its most benign, sleep deprivation is something most of us deal with on a fairly regular basis, while at its most drastic it can be a life-threatening situation.

Sleep deprivation is common simply as a result of circumstance among people who live busy and stressful lives. Students, particularly, often find themselves experiencing some level of sleep deprivation as they struggle to keep up with heavy workloads and busy social lives. Young children are also likely to experience sleep deprivation, as they generally require more sleep than adults, and often do not get it.

Many people are unable to sleep because of anxiety or stress, as well, and when they do sleep, their sleep may be fitful and frequently interrupted, leading to sleep deprivation. Others may have physical pains or issues such as severe coughing that can cause them to be unable to fall asleep, and so to experience sleep deprivation. In some cases, people may actively pursue sleep deprivation for some of its psychological side effects, which can include hallucinations and surges of mania. Some religious mystics, most famously the Desert Fathers of the early Christian Church, are known to have induced sleep deprivation as a way of experiencing spiritual awakening.

Sleep deprivation can have many negative side effects, both physical and mental. Most immediately apparent during sleep deprivation is a slowing of mental awareness and responsiveness, a general dulling of thought processes. Physical responses are also dulled and slowed, and reflexes may be delayed or dampened. Memory loss and short lapses in memory can occur during sleep deprivation, as can spells of delirium and dizziness. Headaches, fainting spells, and nausea are physical effects that can be triggered by sleep deprivation, while a general moodiness and sometimes even psychotic behavior are psychological effects that can accompany a lack of sleep.

People who are experiencing sleep deprivation may also find it very difficult to stay awake, finding themselves constantly drowsy and nodding off. It can be difficult to keep one’s eyes open while sleep deprived, which, coupled with slowed reflexes, makes driving or other high-attention tasks quite dangerous. The body also functions more poorly on an autonomic level without enough sleep, and wounds can take longer to heal, and the body is more prone to sickness due to a weakened immune system.

Just how long people can go without sleep is a somewhat open question. There are people throughout the world who claim to have gone for many years, or even decades, without sleeping. Some of these people even claim to experience none of the usual side effects of sleep deprivation. It is possible that some of these people are actually sleeping in extremely short bursts, called microsleep, which they do not remember, but which does rest their body to prevent sleep deprivation. In the West, the longest officially documented period of sleep deprivation is just over eleven days, recorded in the Guinness Book of World Records in 1965. Because of the health risks associated with such extreme sleep deprivation, however, the Guinness Book of World Records no longer recognizes the category, and since then no one has been documented at staying awake more than eleven consecutive days.