Purpose: Tear film hyperosmolarity is considered one the core mechanism of the dry eye along with the tear film stability. Many tear physiological variables oscillate during the day. This study was designed to assess the differences in tear film osmolarity between morning and afternoon in a group of healthy subjects.

Materials and methods: A total of 25 healthy subjects who fulfilled the study’s inclusion criteria were enrolled for the study. Tear osmolarity was measured using the TearLab™ system in two separated sessions, at 9.30 am and 6.30 pm. A paired t-test and a Bland–Altman test were used to assess the differences between sessions.

Results: Tear osmolarity (mean ± SD) was 309.96 ± 9.00 and 296.48 ± 12.98 mOsm/l at 9.30 am and 6.30 pm, respectively, being significantly lower at 6.30 pm than at 9.30 am (mean difference ± SD = 13.48 ± 8.69 mOsm/l; paired t-test; p < 0.001).

Conclusions: Tear film osmolarity does appear to have some influence by the time of day in healthy patients.