When I get tired, why do I feel it in my eyes?

They tire also. When an individual is tired, the likelihood is that they have also "tired" their eyes out as well. When you read or look at the computer, for example, you do not blink as frequently as normal. After hours of doing this the eyes become dry, creating a tired, heavy sensation. Lubricating eye drops can relieve the sensation, but if you are that tired, a good nap is probably a better option!
Tired. When tired, the eyes often feel tired because your eyes dry out and your eyelids may feel heavy. Try using some artificial tears, take a break from the computer, and close your eyes for a few minutes.