That happens because of the extracellular fluid that flows out of our cells. It equates a third of the water we have in our bodies and, when we lie down at night, that fluid is distributed all over the body, including our face. This causes the swelling, especially in our eyelids. Oh, and the position in which we sleep influences the flowing of the liquid - if you lie with your face down, for example, your whole face will swell. If you sleep on your side, it is possible that one half of your face swells and the other not. Very sexy. Other factors may interfere in the swelling, such as circulatory or renal problems, and the consumption of alcoholic beverages or salty foods.