It is for sure normal to get blurred vision when you are tired. Vision is a very active and complex process , involving the eye, eye muscles, eye nerves working together on the brain. When you are tired, your brain and all the involving parts that ensure you a clear vision certainly will be affected and seem work as well as previously. So we say tiredness resulting from the deprivation of sleep and rest or any other activities can make you feel difficult in seeing objects clearly. For you second question, blurred vision can also be a sign of eye disease. Myopia, far-sightedness, presbyopia, cataracts, glaucoma are all possible causes of blurred vision. My suggestion is you go to have your eyes examined in the ophthalmologist.
Well, the most important cause of it is nerves in brain tired. Eh, it sounds weird. But it is true that nerves in brain will strike with unreasonable and excessive using. I arrived some conclusions about your condition after a deep search online. Firstly, you may be anemia and malnutrition which are easy to reduce oxygen to brain, and then you'll feel dizzy and breathe hard and cannot see things clearly. Secondly, you may use brain excessively so that the nerves that govern eye movement and body system cannot give you a sign of watching. Generally speaking, fatigue can slow down the brains, especially the nerves that conduct your movements, then you cannot catch something quickly, then two images will happen or vague images occur. So you feel blurred vision when tired. But I suggest that you visit a doctor to determine the real reasons through eye exam. By the way, the body is the capital of revolution. Do exercised and take a good rest.
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.