Gravity in the eye—how 'gravitational ischemia' in the retina may be released and resolved through rapid eye movement (REM), a component of gravity-opposition physiology

In normal terrestrial physiology, the superior retinal hemisphere receives significantly more blood flow than the inferior hemisphere—this includes the influence of the cilioretinal artery (which is congenitally present in 30% of the population), and which significantly localizes to the superior temporal retinal quadrant. This may be an evolutionary adaptation to protect the blood circulation in the superior temporal retinal quadrant, which is exposed to gravitational ischemia during sleep, when the eyes are typically rotated up and out—rolling the superior temporal retinal quadrant into the 'bottom' position, when sleeping in supine position with the nose up. Many of the cardinal features of Spaceflight Associated Neuro-Ocular Syndrome (SANS) may potentially be explained as features of gravity-opposition physiology, which become unopposed by gravity during spaceflight. Gravity-opposition physiology may, in fact, create significant challenges for humans involved in long-duration space travel (long-term microgravity). Possible solutions may include the use of artificial gravitational fields in space, such as centrifuges.