Time for eye exam. Tired eyes are often a sign of either dry eyes or eye strain. Both can be related to long hours spent on a repetitive near task (like being on the computer for hours) and can also be caused by a need fro prescription glasses. Occasional blurred vision can be related to dry eyes or prescription changes also. You would be well off having an eye exam to be sure.
Dry eyes. There can be many reasons, however the most common reason 99% of time would be dry eyes or ocular surface disease.