A mind filled with fear and bad thoughts is an unquiet mind. When you are worried, it can be hard to direct your attention on even a minor problem. For some people, this can interfere with work, sleep and other activities. In others, it could lead to a serious mental disorder. People who suffer from anxiety, a nervous disorder, have unreasonable thoughts and fears about the future. Often they are unable to control these thoughts. They are also less able to deal with stress.