TitleResults: The tear film upon wakening (264 6 14 mOsm/L) was hypoosmotic compared with baseline (297 6 15 mOsm/L, P , 0.001). TO (in mOsm/L) at 20 minutes, 40 minutes, 1 hour, 2 hours, 4 hours, and 8 hours were 287 6 10, 292 6 16, 293 6 12, 292 6 10, 289 6 10, and 286 6 10, respectively. CCT (mean 6 SD) at baseline was 552.2 6 35.9 mm and increased to 572.0 6 38.7 mm after sleep. CCT returned to baseline thickness 4 hours after awakening (P , 0.000) and remained stable throughout the day. A small but statistically significant association was found between higher TO and lower CCT (P , 0.0001) and between lower baseline TO and higher PRPH (faster deswelling; P , 0.0001). Conclusions:The diurnal pattern of TO has been established. The association of TO with corneal thickness and deswelling suggests that the tear film tonicity may be partly responsible for corneal hydration control; however, the effect may not be of clinical significance in a normal study cohort.