I have Sjogren's syndrome, Dry eyes, dry mouth. Sjogrens is a disease that attacks the immune system. I have always to be drinking water, my teeth rotted away because I have no saliva, I got corneal ulcers as my eyes were so dry.
The problem with dry eyes is that we use the computers too much or our smart phones and we do not blink enough but it took the doctors two years to finally agree that I had Sjogren's syndrome. Go online and read all about it.
If you just have dry, tired eyes, see your GP and let them prescribe you some eye drops as the ones you can buy over the counter may not be strong or thick enough to keep your eyes moisturised. Or go to the optician for your eye test and tell them, they will tell you how bad your eyes are and then refer you to an eye specialist.