A、Severe headache. B、Too much pleasure in activities. C、High energy levels. D、Thoughts of suicide. D讲座提到，临床抑郁症(clinical depressi

Some colors that people see late at night could cause signs of clinical depression. That was the finding of a study that builds on earlier study findings. They show that individuals who live or work in low levels of light overnight can develop clinical depression.
    Doctors use the words "clinical depression" to describe severe form of depression. Signs may include loss of interest or pleasure in most activities, low energy levels and thoughts of death or suicide.
    In the new study, American investigators designed an experiment that exposed rats to different colors. The researchers chose rats because they sleep during the day and are active at night. The animals were separated into 4 groups. One group of rats was kept in the dark during their nighttime period. Another group was placed in front of a blue light, a third group slept in front of a white light. While a fourth was put in front of a red light. After four weeks, the researchers noted how much sugary water the rats drank. They found that the more depressed animals drank the least amount of water.
    Randy Nelson heads the Department of Neuroscience at Ohio State University. He says animals that slept in blue and white light appeared to be the most depressed.
    Randy Nelson notes that photosensitive cells in the eyes have little to do with eyesight. He says these cells send signals to the area of the brain that controls what has been called the natural sleep-wake cycle. He says there’s a lot of blue in white light, this explains why the blue light and white light make rats appear to be more depressed than the ones that see red light or darkness.
    Mr Nelson has suggestions for people who work late at night, or those who like to stay up late.
    If these people are just living a typical mostly active life during the day, mostly inactive at night, they want to limit exposure to TVs which are quite bluish in the light they give off, and computer screens and things like that. They can get filtered glass. They can get filters on computer screen and filters on eReaders to put it more in the reddish light.
19. What is the sign of clinical depression?
20. Why do the researchers choose rats in the study?
21. What kind of light makes the rats most depressed?
22. What is Mr. Nelson’s suggestion for people who work late at night?