听力原文：M: So, what's your trouble? W: Well, doctor, I haven't been feeling well lately. My biggest problem is that I'm having trouble sleeping. M: Have you tried any of the sleeping medications available? W: I've tried one or two, but they don't seem to help. M: How long have you had this problem? W: Three or four months--it's been rather a long time now. M: Are you suffering from an unusual level of stress in your life lately? W: Not more than usual. My biggest worry is the fact I can't sleep. M: Well, some people don't need as much sleep as others. W: But I feel tired all day, so it is a problem for me. M: Have you been experiencing any other symptoms? W: I've also had a lot of indigestion lately. M: Well, you. are a little overweight, aren't you? W: Yes. And I've gained about seven kilograms this year. M: You really should try to lose at least that extra weight. Do you get much exercise? W: No, not very much. M: Physical activity is very important. W: So what do you recommend, doctor? M: I will give you some medication for your insomnia and your indigestion. But I also recommend that you begin a regular exercise program. W: But I have neither the time nor the money to go to a gym. M: You don't need to go to a gym. A simple walking program would be very beneficial for you. I think you'll find that diet and exercise will be very helpful in taking care of your recent complaints. What is the main cause of the patient's trouble in sleeping?