Sleep deprivation is a lack of sleep that is needed to function normally. It can be caused by working night shifts; traveling through multiple time zones; sleep disorders like insomnia; stress; depression; the menopause; scientific studies into sleep and dreaming; or interrogation and torture.
However, the main cause is at the root of our busy modern society. We just don't have enough time to get everything done.
This problem causes American businesses to lose $100 billion in lost productivity each year. Worse, tired workers are thought to be responsible for many major disasters - ranging from the giant oil spillage of the Exxon Valdez, to the nuclear meltdowns of Three Mile Island and Chernobyl.
However, in the realm of science, staying awake for lengthy periods is useful for sleep experiments. Scientists can learn a lot by looking at a person's behavior, as they encounter confusion, hallucinations, cognitive problems and micro sleep.
If you've never gone for more than 24 hours without sleep, you won't be familiar with the effects of long term sleep loss. But sufferers of insomnia will tell you: it is definitely not a pleasant experience.
Whereas insomnia describes chronic sleeplessness, anyone can experience short term sleep loss as a side effect of some other problem. The physical effects of sleep deprivation affect the whole body, including:
Recent studies also show a link between sleep deprivation and obesity. People who are deprived of sleep find it harder to break down sugar, increasing the risk of diabetes and obesity in the long term.
"There is no hope for a civilization which starts each day to the sound of an alarm clock" ~ Author unknown
Interestingly, when they finally manage to sleep, sufferers of insomnia are more likely to have lucid dreams. Perhaps this is because the normal sleep cycles are thrown out of balance and conscious dreams are more easily triggered.
Needless to say, the physical effects of sleep deprivation are wide-ranging and severe. In fact, if you go for long enough without any sleep, the result can be deadly. This is the case for sufferers of Fatal Familial Insomnia, a rare but incurable sleep disorder that causes total sleeplessness and ultimately death.
Scientists use sleep deprivation studies to isolate the functions of sleep. By looking at how our minds and bodies are impaired after little or no sleep, we can see exactly why sleep is vital to us all.
The most famous sleep deprivation studies involve attempts to break the Guinness World Record. In these experiments, volunteers went for many days and nights without any sleep at all. They allowed researchers to monitor them to see the mental and physical effects of sleep deprivation. Here are two of the more prominent world records:
The New York DJ, Peter Tripp, set a world record for sleep deprivation in 1959. He went for 201 hours (8.4 days) without sleep. He spent most of the time in a glass booth in Times Square, and the rest in a hotel room across the street, set up as a laboratory to monitor his reactions.
The stunt produced strange results. After three days, Tripp began to find things hilarious that weren't funny at all. At other times, he became upset for no reason. He was also confused, asking why there were bolts in the window frames.
By day four, he was suffering from hallucinations and paranoia. At first they were simple patterns - like cobwebs on the doctors' faces, or imagining that paint specks on the table were insects.
But soon his hallucinations became 3D. Tripp imagined mice and kittens scurrying around the room...
Eventually, Peter Tripp became psychotic. He rummaged through draws looking for non-existent money. He accused a technician of trying to harm him. He then claimed he was not Peter Tripp but an imposter. Nothing made sense to him.
On reaching his 200-hour target, Tripp was made to stay awake for one final hour while doctors did more tests. They left the EEG in place as he finally closed his bloodshot eyes and entered a deep 13-hour slumber.
He may have been physically restored, but Tripp's family soon noticed a difference in his personality. His wife said he was moody and depressed. He fought with his boss, became involved in the payola scandal, and was fired from his high profile job as a radio DJ. He went on to have four divorces. Some would say the sleep stunt changed Peter Tripp forever.
Randy Gardner beat the world record for sleep deprivation at the age of 17. A typical high school student from California, Gardner stayed awake for 264 hours (11 days) without using any stimulants. His stunt was part of a school science project on sleep patterns.
The effects of sleep deprivation on Randy Gardner included moodiness, problems with concentration and memory, paranoia and hallucinations. After four days, he had the delusion that he was a famous American football player winning the Rose Bowl. He also mistook a street sign for a person.
On the 11th day, he was asked to subtract seven repeatedly, starting from 100. He stopped when he got to 65... and said he had forgotten what he was doing.
Yet later that day, Randy Gardner held a press conference where he spoke without slurring his words and appeared to be in excellent health.
After his marathon, Gardner slept for nearly 15 hours and returned to a normal sleeping pattern within days. Unlike Peter Tripp (who incidentally used stimulants to stay awake) there were no reports of any long term personality changes.
Peter Tripp and Randy Gardner are known as the most famous sleep deprivation studies, because they involved rigorous scientific testing. They are the most widely cited examples of sleep deprivation today. However, a few other Guinness World Record attempts are worth mentioning.
However, if you are thinking about doing a sleep deprivation stunt of your own - forget it. Guinness World Records no longer recognizes this category because long term sleep deprivation is thought to pose serious health risks, both physically and mentally. It could even trigger other sleep disorders.
What's more, sufferers of chronic insomnia could be staying awake for far longer than these record attempts, which would be unfair to them.
What a shame that the UK's Tony Wright didn't know this when he stayed awake for 266 hours in 2007. He thought he was outdoing Randy Gardner, and hadn't even heard of Toimi Soini. A spokesman for Guinness World Records simply said: "People who attempt records should make sure their research is accurate or they may be very disappointed."
To learn more about sleep deprivation, check out End Your Sleep Deprivation - a website created by Dr Dement, the father of sleep medicine, and the students of Stanford Sleep and Dreams. They share their expert knowledge on the science of sleep so you can sleep healthily and live alertly.