Blurry vision is very common for people. There are several possibilities which could lead to the same result. The most common cause should be refractive errors. Most people in the world are suffering from refractive errors nowadays, including myopia, hyperopia and astigmatism. Other causes related to certain condition or problems, for example presbyopia, intraocular surgery side effects, eye infections, inflammations or allergic reactions and so on. In your case, if you always get blurred vision when you are very tired, I might guess there is some problem with the muscles of your eyes. If we try to focus on some objects, up-clost or at distance, we have to overcome certain diopter to get focused. The two eyes needs to work aligned so that we can get focused and have a clear image. But when you are tired or drunk, you might lose powerful control of your muscles which are responsible for rotating eyes, so it became difficult for your two eye to get focused or work aligned. I suggest you to visit an eye specialist to check your eyes diopter. Meanwhile, you can do more eye exercises to strengthen the controlling ability and balance of muscles. Don't prolong the working time of your eyes. Take frequent breaks to relax the muscles, say 15 minutes every two hours.