This question arose a few years ago. At the time I replied as follows (and it may still be true...):

There seems to be no widely-used specific term, perhaps because the effect is seen as trivial and erratic.  Still, it is important; during the day grit, dead cells and other debris accumulate in the tears, which are not just salt water, but form three layers.  Mucoproteins cover the eyeball, curdling protectively round sharp grit to case it in mucus, a middle salty layer is the main liquid part, and an outer, oily layer reduces evaporation.  At night, movements of the eye and closed eyelids stir this orbital midden, massaging solids towards the inner corner of the eyelids.  There the exposed liquid evaporates till the residual sludge forms pellets that you remove harmlessly by sanitary measures next morning.  ---

Gritty environments such as deserts, may damage eye tissues enough to convert your tears into dilute pus.  This dries on the edges of your eyelids, gluing them shut in spite of their waxy coating that normally reduces spillage and keeps their epidermis water-repellent.  It then can be very disconcerting to awaken from a sleep of exhaustion with eyelids sealed shut, so that you think it is still dark.  Soak them open gently, or you may lose some eyelashes in the sand.