We had one guy that only lasted 3 weeks. Why? He kept falling asleep. He said he had insomnia but we come to find out, he had a drinking problem and would hardly sleep at night. He was out drinking most evenings.
There is no way someone can be able to cover up a drinking problem with the excuse of having insomnia. This is because one thing about alcoholic beverages is that they will always expose you that you have taken them after you come to work. Your whole body is going to smell of alcohol.