Astronauts on six-month or longer ISS missions often experience worsening vision, requiring stronger eyeglasses, NASA said. This is likely due to microgravity-induced fluid shifts toward the head and increased intracranial pressure, contributing to Space-Associated Neuro-Ocular Syndrome (SANS), it added. Experiments like 'Thigh Cuff' are now testing countermeasures to combat these fluid shifts.