Body Position, Time of Day Influence AL, Choroidal Thickness and IOP

As these graphs from the study show supine position decreased AL and increased choroidal thickness, regardless of time of day, compared to upright posture. AL showed a relative decrease when the body was in a supine position, regardless of time of day. Even when the study participants remained upright at night, the choroid thickened, providing evidence that diurnal nighttime choroidal thickening is not solely due to a supine body position, but that other diurnal factors also play a role. Photo: Ross A et al. Sci Rep. 2025;15(1):26556. Click image to enlarge.  Choroidal thickness, axial length (AL) and intraocular pressure (IOP) are among the many ocular processes that are known to undergo diurnal rhythms, but the effects of body position are not well understood. Researchers from the University of Houston College of Optometry (UHCO) recently assessed the extent to which body position affects ocular diurnal rhythms of AL, choroidal thickness, inner retinal microvasculature and IOP.Their study, published last week in Scientific Reports, demonstrated that both body position and time of day influence axial length, choroidal thickness and IOP. It found that diurnal variation in AL was attenuated and diurnal variation in choroidal thickness was augmented when the body was in a supine position for two to four hours. IOP did not display a significant change at two or four hours when the body was in a supine position. However, when IOP was measured 10 minutes following transition to the upright position, an acute reduction was observed.In this investigation, participants (n=23, ages 22 to 46) were enrolled in four four-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 OCT and OCT angiography (OCT-A) imaging were performed every two hours. For supine sessions, IOP was also measured 10 minutes after the body position changed to upright.When upright for four hours in the morning, axial length increased 10.0μm; when supine, it decreased 8.7μm. When upright at night, AL decreased 18.3μm and choroidal thickness increased 7.7μm. When supine at night, axial length decreased 37.0μm and choroidal thickness increased 14.1µm. IOP measured 10 minutes after transitioning to an upright position showed an acute decrease. No significant changes in retinal microvasculature were observed with body position or time.Similarly, mean arterial pressure and mean ocular perfusion pressure showed acute changes after transitioning from supine to upright. “These findings confirm that posture can modulate, but not eliminate,” the influence of diurnal rhythms on IOP, AL and choroidal thickness, “with axial length decreasing and choroidal thickness increasing when the body is in the supine position, such as during sleep,” says study coauthor Lisa Ostrin, OD, PhD, an associate professor at UHCO. She also notes that, while the study was not designed to assess myopia progression, “the results suggest that extrinsic biomechanical factors may influence ocular measurements relevant to refractive development.” These dynamics should be considered when interpreting IOP and biometric data in both clinical and research settings, Dr. Ostrin emphasizes.“Findings related to AL and choroidal thickness may have implications in what is known about circadian modulation of ocular growth signals,” the study authors wrote in their paper. “Because choroidal thickness changes are hypothesized to act as a biometric signal influencing scleral remodeling, postural effects may alter circadian mechanisms that regulate axial growth.”When assessing IOP, the researchers suggested that a patient’s body position should be standardized and documented. Variability in posture during measurements could lead to misinterpretation of IOP, potentially affecting diagnosis and treatment decisions. Click here for the journal source. Ross A, Ostrin LA. Influence of body position and time of day on ocular rhythms. Sci Rep. 2025;15(1):26556.  This article was developed by the editorial staff in conjunction with experts in the field. In the process, AI may have been among the editorial tools used to meet the goals of human editors, who approved all content.