By: Natalie Wolchover
Published: 08/13/2012 09:21 AM EDT on Lifes Little Mysteries
One hears epic accounts of people surviving bullets to the brain, 10-story freefalls or months stranded at sea. But put a human anywhere in the known universe except for the thin shell of space that extends a couple of miles above or below sea level on Earth, and we perish within minutes. As strong and resilient as the human body seems in some situations, considered in the context of the cosmos as a whole, it's unnervingly fragile.
Many of the boundaries within which a typical human can survive have been fully established; the well-known "rule of threes" dictates how long we can forgo air, water and food (roughly three minutes, three days and three weeks, respectively). Other limits are more speculative, because people have seldom, if ever, tested them. For example, how long can you stay awake before you die? How high in altitude can you climb before suffocating? How much acceleration can your body withstand before it rips apart?
Experiments over the decades — some intentional, others accidental — have helped stake out the domain within which we, literally, live.
How long can we stay awake?
Air Force pilots have been known to become so delirious after three or four days of sleep deprivation that they crash their planes (having fallen asleep). Even a single all-nighter impairs driving abilities as much as being drunk. The absolute longest anyone has voluntarily stayed awake before nodding off is 264 hours (about 11 days) — a record set by 17-year-old Randy Gardner for a high-school science fair project in 1965. Before falling asleep on day 11, he was essentially a vegetable with its eyes open. [Top 10 Spooky Sleep Disorders]
But at what point would he have died?
In June, a 26-year-old Chinese man reportedly died 11 days into a sleepless attempt to watch every game of the European Cup. But he was also drinking alcohol and smoking throughout, making it difficult to ascertain his cause of death. No human has ever definitively died from lack of sleep alone, and for obvious ethical reasons, scientists can't find the breaking point in the lab.
They've done it with rats, however. In 1999, sleep researchers at the University of Chicago put rats on a rotating disc positioned over a pool of water, and continuously recorded the rats' brainwaves with a computer program that could recognize the onset of sleep. When the rats nodded off, the disc was suddenly rotated to keep them awake by bumping them against the wall and threatening to knock them into the water. The rats consistently died after two weeks of this misery. Before perishing, the rodents showed symptoms of hypermetabolism, a condition in which the body's resting metabolic rate speeds up so much that it burns excessive calories even while completely still. Hypermetabolism has been tied to lack of sleep. [The 6 Craziest Animal Experiments]
How much radiation can we absorb?
Radiation poses a long-term danger because it mutates DNA, rewriting the genetic code in ways that can lead to cancerous growth of cells. But how much radiation will strike you dead right away? According to Peter Caracappa, a nuclear engineer and radiation safety specialist at Rensselaer Polytechnic Institute, 5 and 6 Sieverts (Sv) over the course of a few minutes will shred up too many cells for your body to fix at once. "The longer the time period over which the dose is accumulated, the higher that range would be, since the body works to repair itself over that time as well," Caracappa told Life's Little Mysteries.
As a point of comparison, some workers at Japan's Fukushima nuclear plant absorbed 0.4 to 1 Sv of radiation per hour while contending with the nuclear disaster last March. Although they survived in the short term, their lifetime cancer risk increased, scientists have said.
Even if one steers clear of nuclear disasters and supernova explosions, the natural background radiation we all experience on Earth (from sources like uranium in the soil, cosmic rays and medical devices) increases our chance of developing cancer in a given year by 0.025 percent, Caracappa said. This sets a bizarre upper limit on the human life span.
"An average person … receiving an average background radiation dose every year over 4,000 years, in the absence of all other influences, would be reasonably assured of contracting a radiation-induced cancer," Caracappa said. In short, even if we eventually manage to eradicate all disease and switch off the genetic commands that tell our bodies to age, tough luck: We will never live past age 4,000.
How much can we accelerate?
The rib cage protects our heart from a hard thump, but it's flimsy security against the kinds of jostling that technology has made possible today. Just how much acceleration can our organs tolerate?
NASA and military researchers have made strides in answering that question for the purposes of safe spacecraft and aircraft design. (You don't want astronauts blacking out during liftoff.) Lateral acceleration — jerking to the side — does a number on our insides because of the asymmetry of the forces. According to a recent article in Popular Science, 14 Gs of lateral acceleration can tear your organs loose from one another. Head-to-foot motion, meanwhile, plunges all the blood to the feet. Between 4 and 8 longitudinal Gs will knock you out. (A force of 1 G is the normal force of gravity we feel here on terra firma, while 14 Gs equals the pull of a planet 14 times as massive.)
Forward or backward acceleration appears to go easiest on the body, because they allow the head and heart to accelerate together. Military experiments in the 1940s and 1950s with a "human decelerator," essentially a rocket sled that zipped back and forth across Edwards Air Force base in California, suggest we can slow down at a rate of 45 Gs, or the equivalent of the gravity of 45 Earths, and still live to talk about it. At that rate, you slow from 630 miles per hour to 0 mph in fractions of a second over a few hundred feet. We probably turn into a bag of spare parts up around 50 Gs, researchers estimate. [What Would Happen If You Fell into a Black Hole?]
What environmental changes can we handle?
Individuals vary greatly in how well they tolerate departures from normal atmospheric conditions, whether these are changes in temperature, pressure or oxygen content of the air. Bounds of survival also depend on how slowly environmental changes set in, because the body can gradually adjust its oxygen usage and metabolism in response to external conditions. But some rough estimates of our breaking points can be made.
Most humans will suffer hyperthermia after 10 minutes in extremely humid, 140-degree-Fahrenheit (60-degrees-Celsius) heat. Death by cold is harder to delimit. A person usually expires when their body temperature drops to 70 degrees F (21 degrees C), but how long this takes to happen depends on how "used to the cold" a person is, and whether a mysterious, latent form of hibernation sets in, which has been known to happen.
The boundaries of survival are better established for long-term comfort. According to a 1958 NASA report, people can live indefinitely in environments that range between roughly 40 degrees F and 95 degrees F (4 and 35 degrees C), if the latter temperature occurs at no more than 50 percent relative humidity. The maximum temperature pushes upward when it's less humid, because lower water content in the air makes it easier to sweat, and thus, keep cool. [Infographic: Human Comfort Zones]
As attested to by any sci-fi movie in which an astronaut's helmet pops off outside the spacecraft, we don't fare too well with abnormal oxygen or pressure levels. At atmospheric pressure, air contains 21 percent oxygen. We die of anoxia when that concentration drops past 11 percent. Too much oxygen also kills, by gradually causing inflammation of the lungs over the course of a few days.
We pass out when the pressure drops below 57 percent of atmospheric pressure — equivalent to that at an altitude of 15,000 feet (4,572 meters). Climbers can push higher because they gradually acclimate their bodies to the drop in oxygen, but no one survives long without an oxygen tank above 26,000 feet (7925 m).
That's about 5 miles (8 kilometers) up. The edge of the known universe lies some 46 billion light-years farther afield.
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If you've ever drifted off to sleep or just woken up from sleep but were unable to move any part of your body -- spurring a sense that you are frozen in your bed -- you may have experienced sleep paralysis. Sleep paralysis is more common in the seconds to minutes when we're first waking up, whether in the morning or in the middle of the night, Gehrman said. When we are in REM sleep, our muscles are paralyzed so that we don't act out our dreams. But with sleep paralysis, a part of the brain wakes sooner than the rest, giving a sense of wakefulness and alertness -- even though the body's muscles are still paralyzed, Gehrman explained. However, <a href="http://www.stanford.edu/~dement/paralysis.html" target="_hplink">sleep paralysis isn't dangerous</a> despite the unsettling feeling experienced by people who have been through it, according to Stanford University. To decrease the number of sleep paralysis episodes you have, stress reduction, getting enough hours of sleep a night and making sure you have a good sleep schedule could help.
REM Sleep Behavior Disorder
Opposite to sleep paralysis, REM sleep behavior disorder occurs when your brain is in REM sleep but your muscles are acting out your dreams, Gehrman explained. WebMD explains the <a href="http://www.webmd.com/sleep-disorders/guide/rem-sleep-behavior-disorder" target="_hplink">signs of the disorder</a>: <blockquote>Dream-enacting behaviors include talking, yelling, punching, kicking, sitting, jumping from bed, arm flailing, and grabbing. An acute form may occur during withdrawal from alcohol or sedative-hypnotic drugs.</blockquote> Stanford Sleep Disorders Clinic sleep specialist Tracy Kuo, Ph.D., told Everyday Health that this <a href="http://www.everydayhealth.com/sleep/other-disorders/REM-behavior-disorder.aspx" target="_hplink">disorder could potentially be violent</a>, especially if the person is kicking or punching his or her self or partner while sleeping. "Without treatment, it tends to get worse over time," she told Everyday Health. However, there are medications a person can take to help people relax their muscles when they sleep so that they <a href="http://www.everydayhealth.com/sleep/other-disorders/REM-behavior-disorder.aspx" target="_hplink">don't have any muscle activity</a> when they are in REM sleep, Everyday Health reported. REM Sleep Behavior Disorder has also been linked to <a href="http://www.webmd.com/sleep-disorders/guide/rem-sleep-behavior-disorder" target="_hplink">neurodegenerative diseases</a> like Parkinson's disease and multisystem atrophy, and seems to occur several years before these diseases, WebMD reported.
Sleep Walking And Talking
Similarly to sleep paralysis, sleep-walking and sleep-talking occur when part of the brain is awake but the rest of it is asleep, Gehrman said. "With sleep-walking, people are mostly asleep but you're engaging in what are usually kind of basic routine behaviors," Gehrman said. "So typically, people sleep walk and go to the bathroom, or go down to the kitchen and get something to eat, but it's all usually very routine." Because sleep-walking and sleep-talking occur in non-REM sleep, since non-REM sleep only produces bland, boring dreams, whatever the person is saying or acting out is not related to what they may actually be dreaming about, he said. However, Gehrman said that there is not yet a clear answer as to why we say or act out the things we do when we are sleep-walking or sleep-talking. Sleep-walking isn't inherently dangerous, but if a child is prone to sleepwalking, Honaker recommends that parents take safety precautions by locking windows, putting safety latches on doors, etc., so that sleep-walking children don't accidentally hurt themselves.
Bedwetting, also known as enuresis, is defined as involuntary urination by a child who is older than age 5 or 6 (either in the day or night), according to the A.D.A.M. Medical Encyclopedia. Sarah Morsbach Honaker Ph.D., a pediatric sleep psychologist at the University of Louisville, told HuffPost that bedwetting is very common in children, and that most kids outgrow it as they get older, many times without any intervention. Honaker said a possible cause for bedwetting is maturational delay, meaning a child's body hasn't yet matured to maintain bladder control throughout the night. "You wouldn't expect a 2-year-old to be dry throughout the night," Honaker told HuffPost. "For some kids, this ability matures later than others." In addition, some kids may just have a lower arousal threshold, meaning that it takes more to rouse them from sleep if their bladder is full, she said. "Because enuresis is outgrown, there's a tendency in some cases for healthcare providers to make the decision not to treat it," Honaker said. "However, there has been research to suggest it impacts self esteem and can have social consequences." Because of that, she suggests that kids whose lives are strongly impacted by bedwetting to consider an intervention like a bedwetting alarm, which senses moisture and goes off so the child wakes up to go to the bathroom.
Night terrors, also known as sleep terrors, occur more often in children, who tend to outgrow them by adolescence, Honaker said. She also clarified that they are not the same thing as nightmares. "When a child has a sleep terror, they're asleep, so typically what will happen is they will be inconsolable, seem very upset, crying, screaming, and don't even seem to recognize the parent when the parent comes into contact with them," Honaker said. Sleep terrors can go on from 2 minutes to 20 minutes or longer, and they can be very scary for parents, she said, but parents should rest assured that sleep terrors are not at all harmful for the children. "They're asleep, so there's no lost sleep, and we don't see them feeling fatigue the next day," she said. "There's typically no recall, and that's a hallmark with a sleep terror." Honaker said that anywhere from 1 to 6 percent of children will experience a sleep terror, with the typical age of onset being between 4 and 12 years old. For parents, the best thing to do is <em>not</em> wake the child up -- "it can actually make the episode worse because the child doesn't see them as a parent," she said.
Teeth grinding, also known as bruxism, occurs when you <a href="http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002386/" target="_hplink">slide your teeth back and forth</a>, and can occur in both the day and night time, according to the A.D.A.M. Medical Encyclopedia. It can be annoying to sleep partners, and can even lead to joint pain or damage in the area. Teeth grinding is a result of any number of factors, including stress, misaligned teeth, ability to relax and sleeping habits, the A.D.A.M. Medical Encylopedia reported. Gehrman said that sometimes, lowering stress and anxiety can help to reduce teeth grinding at night. To manage teeth grinding -- though it isn't a cure -- people can go to their dentists to get a mouth guard to protect their teeth at night.
Exploding Head Syndrome
Exploding head syndrome is definitely more unusual than some other sleep occurrences like sleep-walking or sleep-talking, Gehrman said. "From descriptions, it's this bizarre experience that it does feel like your head is exploding" because of a loud noise going off in the head, Gehrman said. However, he noted that it's not dangerous. The American Sleep Association <a href="http://www.sleepassociation.org/index.php?p=explodingheadsyndrome" target="_hplink">describes it as</a>: <blockquote> ... Similar to a bomb exploding, a gun going off, a clash of cymbals or any other form of loud, indecipherable noise that seems to originate from inside the head.</blockquote> Gehrman said that there is little research on the phenomenon, but it seems to be a very extreme variation of hypnic jerks -- those weird leg jerks that occur when you're first falling asleep. The American Sleep Association reported that people over age 50 and women are more likely to experience the phenomena, and that it's asso<a href="http://www.sleepassociation.org/index.php?p=explodingheadsyndrome" target="_hplink">ciated with high stress and fatigue</a>.