Human tear osmotic pressure: diurnal variations and the closed eye.

Using a micro-osmometry technique, 450 nonreflex tear samples were collected hourly from six subjects throughout their waking hours, and on initial opening of the eyes following six to eight hours of sleep. While the waking day mean found for this population agreed closely with current values in the literature, considerable variation did occur among individuals, both in their mean as well as in their hour-to-hour levels. The prolonged lid closure condition resulted in a considerable decrease in tear osmotic pressure and was found to be significantly different from the open-eye condition (P less than .001). No significant differences could be shown however between findings for men and women for either condition, and no consistent impact of food or fluid intake could be demonstrated.