Why do your eyes start watering when you're tired?

Fatigue and tearing: There are several reasons that eyes will tear when you're tired. Often, people will rub their eyes which will cause tearing. In addition, when we yawn, our eyes tear as well because of squeezing.

Reflex tearing: Tired eyes get dry, a state which sends an urgent signal to the eye to make more tears. When tired, this system tends to overcompensate and send forth even more tears than necessary thus causing watery eyes. An artificial tear wetting drop used when you begin to get tired can help a lot. Also, taking breaks from reading to blink can help. Good luck. Hope that helps!