In order to understand the causes of dry eyes, it is important to understand what makes up the tears in our eyes. Tears are generally made up of 3 different layers: Mucin layer at the bottom to help the tears attach to the eyes, the aqueous layer in the middle which form the bulk of the tears itself and a lipid layer over the top that protects the tears from evaporating.
Disruption in any of these layers will lead to dry eyes, and this can be due to microbiological infection, inflammation, inadequate nutrition or can also be the side effects of some ocular surgeries.
However, generally dry eyes can be considered to fall in one of two types or a combination. The first type that is most common is called the evaporative dry eye. This is usually because the Meibomian glands that produce the oil layer is not working effectively and thus the tears evaporate too quickly. The second less common type is aqueous dry eye where there is unsufficient production of tears, which is usually associated with a disorder called sjorgen syndrome.