ESCRS 2026: Corneal measurements vary significantly depending on the time of day, study finds

Time of day significantly affects corneal measurements, ESCRS 2026 study finds

A new study presented at the 44th Congress of the ESCRS in London has found that several corneal morphological parameters vary significantly over the course of the day, a finding the investigators say should be taken into account when timing clinical measurements.

Study design

David P. Piñero, PhD, full professor and researcher in the Department of Optics, Pharmacology and Anatomy at the University of Alicante, and colleagues evaluated 109 healthy volunteers over a 10-hour period, taking several consecutive corneal measurements throughout the day.

Key findings

The investigators found a significant reduction in intraocular pressure, minimum corneal thickness, central corneal thickness, and corneal volume during the morning. The posterior corneal surface did not change significantly over the course of the day.

The anterior corneal surface did show statistically significant changes during the day, though Piñero described these as clinically negligible—around 0.25 D in terms of keratometry. A sector analysis showed that these changes occurred mainly within the central 4 mm of the cornea, and specifically in the central infratemporal area rather than the very centre.

The researchers also correlated the daily changes with patient habits, finding associations with morning water and coffee intake and with smoking, as well as correlations between corneal thickness changes and topographic changes occurring during the day. Piñero noted that the relationship with coffee intake was a particularly striking finding, suggesting that caffeine may affect not only behaviour but also morphological and anatomical parts of the eye.

Clinical implications

Piñero said the results highlight the importance of considering the time of day at which corneal measurements are taken in order to support accurate clinical decision-making. He attributed the bias seen in early-morning measurements to eyelid closure during sleep, which he said generates corneal flattening and an increase in intraocular pressure. He cautioned that this bias could lead to incorrect conclusions, particularly in the assessment of certain pathological conditions, and that clinicians should be careful when analysing measurements taken very early in the morning.

Next steps

Piñero said his group has completed—though not yet presented—a similar analysis in patients with keratoconus, and found a markedly different trend: the mechanical weakening of the cornea associated with keratoconus generates daytime changes that may hold diagnostic value. He said this warrants further investigation.