Influence of body position and time of day on ocular rhythms

Abstract

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.

Keywords: Choroid; Circadian rhythm; Intraocular pressure; Posture.

Fig. 1

Protocol for each of the four experimental conditions, AM upright, AM supine, PM upright, and PM supine, which took place in a randomized order for each participant. At each timepoint (baseline, 2 h, and 4 h), IOP, blood pressure, biometry, and OCT/OCTA imaging were performed. For supine conditions, IOP and blood pressure were measured in both the supine and upright positions at 2 and 4 h.

Fig. 2

(A) Mean (± standard error) axial length (mm) over time and (B) changes in axial length from baseline at 2 and 4 h; (C) Choroidal thickness (µm) over time and (D) changes in choroidal thickness from baseline at 2 and 4 h; (E) Corneal thickness (µm) over time and (F) changes in corneal thickness from baseline at 2 and 4 h for four conditions, AM upright (black), AM supine (red), PM upright (black), PM supine (red); measurements were always taken in an upright position; * represents significant changes from baseline; bracket represents significant differences between conditions.

Fig. 3

(A) Mean (± standard error) intraocular pressure (mmHg) over time and (B) changes in intraocular pressure from baseline at 2 and 4 h; (C) Mean arterial pressure (mmHg) over time and (D) changes in mean arterial pressure from baseline at 2 and 4 h; (E) Mean ocular perfusion pressure (mmHg) over time and (F) mean ocular perfusion pressure from baseline at 2 and 4 h for four conditions, AM upright (black), AM supine when measured supine (solid red), AM supine when measured upright (open red); PM upright (black), PM supine when measured supine (solid red), PM supine when measured upright (open red); * represents significant changes from baseline; bracket represents significant differences between conditions.