Fluorescein was loaded into the corneal stroma of 12 subjects. Two hours later, a thick (0.4 mm), low water content contact lens was placed on the cornea and the fluorescence intensity ratio from excitation at 490 and 450 nm was measured through the lens. Lines indicate lens on and off. Although this illustrates an extreme lens, this general effect is observed in even the most gas permeable experimental lenses during sleep. Bonanno J, Polse K. Corneal acidosis during contact lens wear: Effects of hypoxia and CO2. Invest Ophthalmol Vis Sci 1987;28:1514.