Variations of axial eye shortening in response to positive defocus over the day

Purpose Short-term visually induced axial eye shortening and choroidal thickening are of interest since they may be linked to reduced myopia development. We studied whether axial eye shortening, induced by imposing positive defocus, varies throughout the day, and whether emmetropes and myopes differ in their responses. Methods Twenty young adults (10 emmetropes, 10 myopes) completed a daytime protocol conducted between 10:00 am and 3:00 pm, consisting of repeated 30- minute sessions of movie watching while the right eye was myopically defocused by a trial lens, imposing approximately +3D of myopic defocus at the viewing distance used. Each session was started and was followed by a washout period of 15 min. Axial length was measured in both eyes using the Lenstar LS900 biometer immediately before and after each session at five time points. Results Emmetropic eyes showed significant axial length shortening in response to imposed myopic defocus in the morning (10 am: −10.1 ± 7.8 µm, p = 0.0028, 11 am: −9.8 ± 7.0 µm, p = 0.0017) and again in the afternoon (3 pm: −11.2 ± 5.2 µm, p = 0.0001). No significant responses were observed at 1:00 pm and 2:00 pm. Different from emmetropes, myopic eyes did not exhibit significant axial length changes at any time point. Conclusions Emmetropic eyes exhibited time-of-day-dependent variability in short-term axial length responses to myopic defocus, with stronger responses in the morning and afternoon but no responses around midday. The mechanisms underlying choroidal thickening appear to be modulated over the day. Myopic eyes showed minimal or no responses to positive defocus.