Well, it seems that you like to read something with your cellphone right? I quite understand the convenience it brings to you however, you should not neglect the harm it would generate, which could cause some damage to your eyes, leading to an eye strain or decreased vision. Anyway, it is not a good idea, I think natural light is good for reading, maybe you can reading in natural light when it is possible ok?
Well, yes, small screen can be bad for your eyes. According to some researches, the small screen, such as the cell phone, will just do a harm job to your eyes. First, you should know that when you are reading something in the small screen, it will make you to focus on the small prints carefully. In a long time, it will just lead to some eye strain. When you suffer eye strain in your eyes, it will just lead to many other eyes diseases, such as red eyes, bloodshot eyes, even dry eyes. In some cases, it also affects your sight. So you should not read something in small screen.
Yes, it is bad for eyes. More and more people are complaining about their blurred vision and eye pain because of reading on their cell phone. Because the screen of cell phone is smaller than usually book or computer screen, it is easier to get eye strain with looking at it for hours. Using computer for hours will make your eyes red, strained and vision blurred, not to mention the cell phone. Secondly most of us use cell phone anytime at any place. For example, you can find people, looking at the cell phone in his/her hand, walking by you. You might find many people around you reading on cell phones when they are lying in bed, sitting in sofa or eating food at tables. Pro-long working time of eyes and bad gestures could make your eye feel really bad. In the evening, when you are reading on cell phone without light on, it is more harmful to your eyes because the dim light and low screen contrast level. Try not to rely on your cell phone and quit reading on it. Hope your eyes could get better.