Influence of body position and time of day on ocular rhythms

To determine the influence of time of day and body position on axial length, microvasculature, and intraocular pressure (IOP), participants (N = 23, ages 22-46 years) were enrolled in four 4-hour experimental sessions, which varied by time of day (morning or night) and body position (upright or supine). Blood pressure, heart rate, IOP, biometry, and optical coherence tomography (OCT) and OCT angiography (OCTA) imaging were performed every 2 h. For supine sessions, IOP was also measured 10 min after position changed to upright. When upright for 4 h in the morning, axial length increased 10.0 ± 2.8 μm (P = 0.005). When supine in the morning, axial length decreased 8.7 ± 2.2 μm (P < 0.001). When upright at night, axial length decreased 18.3 ± 2.3 μm (P < 0.001), and choroidal thickness increased 7.7 ± 1.4 μm (P < 0.001). When supine at night, axial length decreased 37.0 ± 3.4 μm (P < 0.001) and choroidal thickness increased 14.1 ± 1.7 μm (P < 0.001). IOP measured 10 min after transitioning to an upright position showed an acute decrease (P < 0.05). No significant changes in retinal microvasculature were observed with body position or time (P > 0.05 for all). In conclusion, body position and time of day influence axial length, choroidal thickness, and IOP, indicating that extrinsic factors work in conjunction with diurnal factors to mediate ocular rhythms.