Is there a clock in the cornea?

The overnight corneal swelling response is generally attributed to hypoxia but other factors, such as osmolarity, may contribute. A seldom considered alternative is that the response represents a biological rhythm instigated by the dark/light cycle. The purpose of this study was to investigate that possibility. Ten participants wore goggles designed to exclude light from one eye only, while allowing normal blinking and access to the external atmosphere. Measurements taken from both eyes, prior to and after 6 h of goggle wear, included corneal thickness (epithelial and stromal, centrally and in 4 concentric annuli), intraocular pressure and tear film osmolarity. Data were fitted by linear mixed models to assess the effect of light deprivation. Mean changes (SD) in central corneal thickness for light exposed eyes were 6.8 (8.1) μm in the epithelium and -4.0 (9.3) μm in the stroma. Corresponding values for light deprived eyes were -.1 (10.8) μm and 5.1 (10.6) μm. Corneal thickness changes (epithelial or stromal) between light deprived and light exposed eyes were not significant in any location (p > 0.23). Neither intraocular pressure, nor tear film osmolarity altered significantly (p > 0.6). These data show that monocular darkness does not generate corneal thickness changes, a finding which does not support the hypothesis that corneal thickness control is light mediated in humans. The possibility remains that such a mechanism does exist but contributes only a relatively minor portion to the response and/or has a triggering mechanism that is presently uncharacterised.