Do you often feel lonely? What do you do when you feel that way? Hide yourself away and spend your time reading, watching TV or walking around?

    You might think that feeling lonely is just like feeling happy, sad or scared — that it's just one of your various moods. That is true. However, if you let yourself be lonely for too long without dealing with it, you could be making a serious mistake.

    Doctors have known for some time that feeling lonely is bad for the mind. It can lead to mental health problems such as depression, stress and reduce confidence. "Being lonely means not feeling connected or cared for, but it's not about being mentally alone," Lisa Jaremka, scientist from Ohio State University, US, told Live Science in January. And there's growing evidence that not having friends is connected with physical illness as well.

    In 2006, for example, scientists studied 2,800 women who had cancer. They found that those who had few friends or family were five times more likely to die of their disease than women with many social contacts. Also, even healthy people had a better chance of falling ill if they felt left out by others, according to the BBC.

    The results have scientists thinking that loneliness might hurt the immune system（免疫系统）, which protects the body from diseases.

    Hoping to prove this theory, Jaremka and her research team put volunteers（志愿者） through a stress test. During the test, volunteers were asked to make an unprepared speech in front of a group of stony-faced people. The researchers found that volunteers who said they were lonely in their daily lives felt more stress during the test. And their blood samples showed that all the stress had managed to cause harmful changes to their immune system.

     "Loneliness has been thought of in many ways as a chronic stressor（慢性增压器） — a socially painful situation that can last for quite a long time," explained Jaremka, who led the study.

    The number of people suffering from loneliness is increasing all over the world. However, solving the problem is easier said than done. It won't work to just "tell anyone to go out and find someone to love you", said Jaremka. "We need to create support networks."

—Yes, it fits me very ____.

    After a morning hike in the Saneum HealingForest, 46-year-old firefighter Kang Byoung-wook has tea made from the bark of an elm tree, practices yoga (瑜伽), and makes a picture with dried flowers. He is one of 40 firefighters taking part in a three-day program, the aim of which is to offer "forest healing" (森林治愈); the firefighters all have posttraumatic stress disorder (创伤后应激障碍).

    Saneum is one of three official healing forests in South Korea. Soon there will be 34 more. South Koreans-many of whom suffer from work stress, digital addiction, and great academic pressure have accepted the medicalization of nature with great enthusiasm.

    There is increasing evidence that being outside in a pleasant natural environment is good for us. But how many of us get to enjoy nature regularly? Fewer and fewer, it seems. According to Lisa Nisbet, a psychology professor at Canada s Trent University, evidence for the benefits of nature is pouring in at a time when we are most disconnected from it.

    "We don't think of being outdoors as a way to increase happiness," says Nisbet. "We think other things will, like shopping or TV." But South Korea is starting to challenge this opinion.

    So what are some of the benefits of nature that Nisbet refers to? Being surrounded by nature has one obvious effect: It calms us and reduces our stress levels. This has been shown to lower blood pressure and heart rates.

    Another experiment conducted by psychologist Stephen Kaplan found that people who took a 50-minute walk in a park had better attention and short-term memory than those who took a walk along a city street.

    Perhaps what's more surprising is that nature may also make us more creative. David Strayer, a psychologist at the University of Utah, showed as much with a group of participants, who performed 50 percent better on creative problem-solving tasks after three days of wilderness backpacking.

    In fact, we may never know exactly what nature does to the brain. Something mysterious will always remain, and maybe that's as it should be.