Assessment and impact of the time of day on aqueous tear evaporation in normal subjects

OBJECTIVES: To determine the impact of the time of day on aqueous tear (AT) evaporation in normal subjects on two consecutive days. METHODS: In a controlled laboratory setting, morning and afternoon AT evaporation was tested in 19 normal subjects, at the same time of day on two consecutive days. Evaporometry was used at two ranges of relative humidity (RH) 25% to 35% and 35% to 45%. RESULTS: Mean AT evaporation rates were 0.069 ± 0.024 for 25% to 35% RH and 0.049 ± 0.018 for 35% to 45% (P = 0.001). There were significant differences for both RH between time of day (P < 0.05) on day 1, but not observed on day 2. Variation between days showed no difference for RH during the afternoon, but there was a difference during the morning (P = 0.042). CONCLUSIONS: The data are remarkably consistent between study days, but there is more fluctuation during the morning than in the afternoon. Therefore, to further standardize AT evaporation study protocol, we recommend perform evaporometry measurements during the afternoon rather than the morning, because our results showed less variability during the afternoon test between days.