You may have an uncorrected refractive error or dry eyes. The first thing you should do is get your refractive status checked and corrected with glasses if needed. People above 40 years of age usually need reading glasses.
Make sure u get a good nights sleep regularly..that means nothing less than 8 hours. Sleep is very important for your eyes.
Dry eye may also cause these symptoms. So try to blink more often while reading. Try not to sit with the air conditioning blowing into your eyes (even when you are in a car). Take a break from reading every 20 minutes for about 20 seconds. If all this doesn't work, then consult an Ophthalmologist to evaluate and treat the dry eye. Hope this helps.