It is generally believed that humans can't live without sleep. As any person suffering from lack of sleep knows, dark circles under the eyes usually stand out after a bad night's sleep.

    But why do people get these dark eye bags? The answer is both genetic (that is, relating to your DNA) and environmental (a result of your everyday living, such as rubbing your eyes or getting too little sleep), said Dr. Carol Clinton, a skin-care researcher in Dublin, Ohio.

    But “the biggest reason is genes,” Clinton told Live Science. Eye bags are generally more noticeable in people who, because of genes, have thin or pale skin. When people are tired or very nervous, blood circulation in the eye area becomes slow, allowing blood to pool there, Clinton said. Capillaries（毛细血管）stand out, leading to dark eye circles, she said.

    Besides, eye bags result from eyes' fat moving forward. An age-connected increase in fat beneath the eyeballs leads to eye bags. “Anyway, when your eyes' fat is moving forward, you may think about having an operation in that area,” Clinton said, “A person can get 9 hours of sleep a night, but still have eye bags because of a genetic development.”

    In other cases, environmental problems cause eye bags. For example, allergies（过敏）—especially seasonal allergies—can cause capillaries to stand out. Getting too much sun can also damage and thin the skin, making dark circles under the eyes easy to see. Besides, eating salty foods can cause the body to hold more water, which makes eye bags worse, according to the Mayo Clinic.

    Melinda Skaar wasn't expecting any phone calls. Skaar was working late in her office at the First Internet bank of California. By 10:45 that night she was almost ready to go home when the phone rang. Picking it up, she heard a guard shouting, "There is a fire! Get out of there." Skaar didn't panic. She figured that it was just a small fire. Her office building was huge. There were 62 floors and her desk was on the 37th floor. Skaar called out to office mate Stephen Oksas, who also stayed late to work. But when they got out to the hallway, they were met by a cloud of black smoke. Rushing back, Skaar shut the door and filled the space at the bottom of the door with her jacket to keep the smoke out.

    Then they called 911. Before they could call their families, however, the line went dead. That meant that they were completely cut off from the outside world. All they could do was wait and hope someone would come to rescue them.

    Minutes ticked by. Smoke began to float into the office. Soon it became hard for them to breathe. Looking around, Skaar noticed a small workroom. It seemed to have cleaner air. So they crowded there. That helped for a while, but in time even the workroom was filled with deadly smoke. Hopeless, they tried to break the windows, but the glass was not breakable. Everything they threw at it just bounced back. Defeated, they struggled back to the workroom. They felt weak and dizzy. Soon Skaar found Oksas had passed out.

    As Skaar and Oksas lay near death, rescuers were rushing to find them. At last, at about 4 a.m., firefighters found them.

    Skaar and Oksas knew they were lucky to be alive. Sunday is my birthday, Skaar told a reporter. She would be turning 29, but she knew she had already got the best present possible—the gift of life.

    My Year Abroad

    This month in Travelers Corner there are three teenagers' experiences in year-abroad programmes.

    Mariko Okada - Tokyo

    My year abroad in the United States was a fantastic experience. I'm not a shy person, and I was very comfortable speaking to everyone. So I got lots of speaking practice. I also learned lots of interesting things about American culture. When I got home, my friends all said that I had improved so much! I hope to go back again in the future.

    Carla Fonseca - Rio de Janeiro

    I spent last year studying English in London. I'm from a small town, and London is a very big city. Sometimes I felt it was too big. There were so many people to talk to, but I always felt bad about my English. I missed my family, and I really missed my two cats. My roommate was always using our telephone, so I hardly had the chance for a nice long talk with my parents. I think it was a good experience for me, but I'm glad to be home!

    Alvin Chen - Hong Kong

    Studying in New Zealand was an interesting experience for me, but it was also lots of hard work! I had English classes six hours a day, five days a week—with lots of homework. I also kept a diary of my experience. I like to write, and I wrote two or three pages in my diary every day. On Saturdays, my home stay family took me to lots of interesting places and showed me so many wonderful things about the culture. I'm really glad I went!

    Professor Martin's report says that children who attend a number of schools, because their parents have to move around the country, probably make slow progress in their studies. There are also signs, says Professor Martin, that an unusually large number of such children are mentally affected.

    The professor says, "It's true, my personal feeling is that children should stay in one school. Our feelings are based on research and not on any personal feeling that I or many assistants may have on the subject."

    Captain Thomas James, an army lecturer for the past 20 years and himself a father of two, said, "I've never heard such rubbish. Taking me for example, no harm is done to the education of my children who change school regularly—if they keep to the same system, as in our Army school. In my experience—and I've known quite a few of them—Army children are as well-adjusted (适应) as any others, if not better. What the professor doesn't appear to appreciate is the fact that in such situations children will adapt (适应) much better than grown-ups."

    When this was put to Professor Martin, he said that at no time has his team suggested that all such children were backward or mentally affected in some way, but simply that in their experience there was a clear tendency (倾向).

    "Our findings show that while the very bright child can deal with regular changes without harming his or her general progress in studies, the majority (大多数) of children suffer from constantly (不断地) having to enter a new learning situation."

    That people often experience trouble sleeping in a different bed in unfamiliar surroundings is a phenomenon known as the 4Tirst-nighf, effect. If a person stays in the same room the following night they tend to sleep more soundly. Yuka Sasaki and her colleagues at Brown University set out to investigate the origins of this effect.

    Dr. Sasaki knew the first-night effect probably has something to do with how humans evolved.

    The puzzle was what benefit would be gained from it when performance might be affected the following day. She also knew from previous work conducted on birds and dolphins that these animals put half of their brains to sleep at a time so that they can rest while remaining alert enough to avoid predators (捕食者). This led her to wonder if people might be doing the same thing. To take a closer look, her team studied 35 healthy people as they slept in the unfamiliar environment of the university's Department of Psychological Sciences. The participants each slept in the department for two nights and were carefully monitored with techniques that looked at the activity of their brains. Dr. Sasaki found, as expected, the participants slept less well on their first night than they did on their second, taking more than twice as long to fall asleep and sleeping less overall. During deep sleep, the participants' brains behaved in a similar manner seen in birds and dolphins. On the first night only, the left hemispheres (半球) of their brains did not sleep nearly as deeply as their right hemispheres did.

    Curious if the left hemispheres were indeed remaining awake to process information detected in the surrounding environment, Dr. Sasaki re-ran the experiment while presenting the sleeping participants with a mix of regularly timed beeps (蜂鸣声) of the same tone and irregular beeps of a different tone during the night. She worked out that, if the left hemisphere was staying alert to keep guard in a strange environment, then it would react to the irregular beeps by stirring people from sleep and would ignore the regularly timed ones. This is precisely what she found.