It’s because when we are tired, we don’t produce enough moisture our eyes need, and rubbing them stimulates the tear glands. They are called lachrymal glands, one of which lies above each eyeball. They continuously produce tears that are necessary to keep the eyes moist and clean. As you blink about 15 times a minute, the eyelid washes tears across the eye. Surplus tear fluid drains down the tear ducts to the nose.
If your eyes feel dry and itchy several times a day you might have dry eye syndrome. This may be because eyes do not produce enough tears or because your tears are evaporating too quickly. Some people naturally produce more tears than others, and those with lower tear production are more likely to have eye irritations. Persistent dryness may be caused by a thyroid condition called Hashimoto’s disease, or a glandular disease called Sjogren’s syndrome.
In most cases, however, dry eyes result from an inadequate fluid coming out of the lachrymal glands. This happens especially when you are tired. The best treatment is not to pick up kitchen knife and cut onions or have another viewing of tragedy-filled Titanic but to rub your eyes to activate the tear glands. Drinking plenty of water throughout the day might also help to keep your eyes lubricated.
Lacrimal gland (Wikipedia)