Basically, sleep deprivation is when one doesn’t get enough sleep. When someone is in a chronic sleep-restricted state they’ll notice excessive daytime sleepiness, fatigue, clumsiness, and weight gain or weight loss. In addition, being sleep-deprived affects both the brain and cognitive function.

Unlike other basic bodily functions, such as eating and breathing, we still do not fully understand why people need to sleep. There are theories, some think sleep may be the process by which the brain shuts down so it can store the day’s memories. Others, like Dr. Joyce Walseben, a psychiatrist and the former director of Bellevue Hospital’s Sleep Disorders Center, point to sleep’s importance in regulating the body’s hormones. But these theories are not complete.

Sleep deprivation is nearly as misunderstood as sleep itself, but it can physically and mentally harm people in myriad ways. Losing sleep can cause hallucinations, psychosis, and long-term memory impairment. Some studies have linked sleep deprivation to chronic conditions like hypertension, diabetes, and bipolar disorder. In 2003, neurologists at the University of Pennsylvania found that sleep deprivation over three consecutive nights (in the study, staying awake for 88 hours) as well as chronic sleep loss (in the study, four to six hours of sleep each night for 14 nights) seriously impaired cognitive functions in healthy adults. Also in 2003, Japanese researchers found that total sleep deprivation can cause high blood pressure and has “profound” effects on the immune system.

In 1989, at the University of Chicago, researchers observed rats which died after being kept awake non-stop for several weeks. (According to a 2009 article, specialists who have looked at the 1989 study dispute which effects of sleep deprivation ultimately killed the rats. It could have been hypothermia brought on by decreased body temperatures, illnesses that arose from damaged immune systems, or severe brain damage.) In July 2012, Chinese soccer fan Jiang Xiaoshan died after staying awake for 11 days to watch all of the European Football Championship. In August, a Bank of America intern died after three days of sleep deprivation.

There’s a subset of cases whereby sleep deprivation can actually lead to an enhanced mood, alertness, and increased energy. Note that relatively few studies have compared the different effects between chronic partial-sleep restriction and acute total-sleep deprivation, and the total absence of sleep over long periods of time has not been studied in humans.

When rats were exposed to prolonged sleep deprivation the result was that both food intake and energy expenditure increased, resulting in a net weight loss, and ultimately leading to death. The hypothesis of this study is that when moderate chronic sleep debt goes hand-in-hand with habitual short sleep, energy expenditure and increased appetite are encouraged; and, in societies where high-calorie food is freely available the equation is tipped towards food intake rather than expenditure. Nationally representative samples used in several large studies suggest that one of the causes of the United States obesity problem could possibly be due to the corresponding decrease in the average number of hours that people sleep. These findings indicate that the hormones that regulate appetite and glucose metabolism could be disrupted because of sleep deprivation. It appears that the association between obesity and sleep deprivation is strongest in young and middle-age adults. On the other hand, there are scientists who believe that related problems, such as sleep apnea, together with the physical discomfort of obesity, reduce a person’s likelihood of getting a good night’s sleep.

There have also been studies that show sleep restriction might have

potential when it comes to treating depression. We know that people suffering from depression experience earlier incidences of REM sleep, plus increased rapid eye movements; and monitoring a patient’s EEG and waking them during bouts of REM sleep appears to produce a therapeutic effect, thus alleviating symptoms of depression.

When sleep deprived, up to 60% of patients show signs of immediate recovery; however, most relapse the next night. It’s believed that this effect is linked to increases in the brain-derived neurotrophic factor (BDNF). It’s also been shown that, in normal people, chronotype is related to the effect that sleep deprivation has on mood: following sleep deprivation, people who prefer mornings become more depressed, while those who prefer evenings show a marked improvement in their mood. In 2014, a thorough evaluation of the human metabolome in sleep deprivation discovered that 27 metabolites are increased following 24 waking hours, with suggestions that tryptophan, serotonin, and taurine may be contributing to the antidepressive effect.

How long can a person stay awake? In 1965, Randy Gardner stayed awake for 11 days as part of an experiment.

If you are struggling with sleep deprivation symptoms, you should talk to your physician about treatment options.