What's in a coffee....

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I used to have two or three cups of coffee a day, one in the morning, one at noon and one before sleeping. But sometimes, I cannot sleep and my friends tell me it is because of the coffee I drink before sleeping. I have stopped drinking coffee recently only because I was advised that coffee will worsen illnesses such as kidney stones and arthritis. What is in coffee that makes one not able to sleep? They also say that drinking coffee before taking an exam makes one active which makes better exam results. I don't understand what's in a coffee. I am really so confused about this drink. Some advise it as a healthy drink while others say it is bad for the health. Sometimes I am so confused reading about what coffee could do to our body. Anyone out there knows about coffee? Thanks...