Measurement of the overnight corneal swelling response is one of the main indicators of the suitability of a new contact lens material for extended wear. One potential problem with overnight corneal swelling studies, however, is the failure to control certain variables that can significantly weaken the statistical power of the results. We investigated the effect of subjects’ lens wearing history, or “adaptation” level, on the magnitude of the overnight corneal swelling response. In this study, the overnight corneal swelling response of subjects who were considered non-wearers, daily wearers, or extended wearers was measured while wearing a thin, low water-content hydrogel lens, and on another occasion, with no lens in place. The lenses were of 38.6% water content material, 0.035 mm in center thickness, and −1.00 D to −6.00 D in power. The mean overnight corneal swelling results with the lens in place were 11.3% (±3.3), 9.3% (±2.8), and 7.4% (±2.3) for 13 non-wearers, 14 daily wearers, and 14 extended wearers, respectively. In a second study, the mean overnight corneal swelling results with no lens in place were 3.8% (±1.3), 2.0% (±2.6), and 0.7% (±2.2) for 13 non-wearers, 13 daily wearers, and 13 extended wearers. There was no significant difference in baseline corneal thickness between the three groups in either of the two studies. These results indicate that it is not valid to compare the overnight corneal swelling response of an investigational contact lens to the widely accepted no-lens overnight corneal swelling value of approximately 4%, unless the subjects used in that particular study were known to be non-wearers, since this may indicate an artificially superior physiological performance.