Eye doctors know that intraocular pressure varies throughout the day—but that’s not the only ocular factor that experiences diurnal changes. According to newly published research, the human eye’s response to defocus can vary depending on an unexpected factor: the time of day. An Australian-based research team’s study shows that evening exposure to myopic defocus causes a larger reduction in axial length, while morning exposure to hyperopic defocus increases it. The findings also show axial length changes are associated with opposite changes in choroidal thickness.

The team looked at the effects of monocular myopic and hyperopic defocus on axial length and choroidal thickness when applied in the morning (between 10am and noon) vs. in the evening (5pm to 7pm) in adults. They repeated axial length (using Zeiss’s IOL Master) and choroidal thickness (using an optical coherence tomographer) measurements over the course of three consecutive days. What they found was that exposure to myopic defocus caused a significant reduction in axial length and thickening of the subfoveal choroid at both times. They also noted that, compared with baseline data, the relative changes in axial length and choroidal thickness with defocus were significantly greater for evening exposure than morning.

“These findings represent a potential interaction between the signal associated with the eye's natural diurnal rhythm and the visual signal associated with the optical defocus, making the eye perhaps more responsive to hyperopic defocus (or ‘go’ signal) in the morning, and to myopic defocus (or ‘stop’ signal) in the latter half of the day,” the researchers concluded in their paper.