The reason why your eyes feel heavy and tired may caused by lacking of sleep. When you don't have enough sleep, your eyes muscles are stay excited as well, so our eyes will feel tired and heavy. If you want to improve this situation, what you do first is to have enough sleep. Moreover, you can use a cold washcloth to give your eyes compresses. Doing eye exercise is good for your eyes as well. When you get tired after doing reading or focusing on a point about 1 or two hours, you should close your eyes and have a rest. You can use eye drops to help your eyes get well.
First check you routine to make sure that it's not because you use your eyes too much and do not get enough rest. Try to take more rest and sleep more, use cold compresses and do eye exercises. If your eyes still feel heavy and tired, use some eyedrop. Sometimes the reason for you to feel eye tired is because your eyes are dry and do not get enough water. But if reasons above are not your reason, you'd better go to see your ophthalmologist to make a check. Maybe your problem is caused by infection and then you need professional treatment.