What's the reason for my eyes getting tired and achy?

Consider dry eye: "tired eyes" are often actually dry eyes, which tend to be more symptomatic later in the day. Artificial tears several times daily are a good start for treatment.

Likely dry eye: Your symptoms are likely caused by dry eye. Try wan compresses 1-2 times a day, frequent artificial tears during the day, ointment at night, and omega-3 fatty acid supplements. If no improvement, go see an eye doctor for further evaluation and treatment.