Ask any heavy reader- they'd probably have had several bad postures. When you are reading The Godfather, it's simply unputdownable. You cannot leave the story and that means you stretch along hours without moving beyond the four by four feet of space that you settle in. But, we usually toss, and turn, and lie down and sit up, and do some half-up-half-down postures, all within the same the square of area.
I have sat for hours in the same hunched up position, a book in my hand, neck bent down towards the book in a posture as unhealthy as possible, looking down for hours together immersed in the world of fiction with just the fingers turning one page after another, without realizing the strain that it's putting on my neck and spine. All that is left would be a satisfying read and a stiff neck. Bad posture- it's a perennial issue if not consciously rectified.
My mom has always been the one to correct me on my posture while reading, and make sure I sit well and comfortable. Nowadays, I try to be self-conscious and settle down in a position where I won't be slumped in a baleful posture for hours. It is a very important thing. As much as it is fun to relate to memes that show the different postures of bookworms, it is nothing to be proud of. Sitting erect and comfortable without straining any part of your body is absolutely necessary to avoid unpleasant effects of it in future.
So, next time you sit down to read, make yourself comfortable and ensure that you are in a good posture. This actually applies to anything you do, not just reading! Sit straight, and don't slump. Keep your hunches limited to your guesswork on the plot, don't extend it to your posture.