Considerations about light and colour for human beings in microgravity conditions

The race to long-term space missions beyond Earth's atmosphere requires a deep understanding of how environmental factors, such as light and colour, affect human physiology and psychology in microgravity. While microgravity has been extensively shown to compromise multiple aspects of human health, including the musculoskeletal and ocular systems, its impact on visual perception, specifically colour perception, remains an area of limited investigation. This paper outlines the current state of research, highlighting the need for dedicated studies on the perceptual and physiological responses to light and colour stimuli in microgravity. Existing studies indicate that microgravity significantly alters the integrity of the human body. Visual stimuli have been proposed and tested as countermeasures against space-induced stress in simulated space station environments. For instance, studies using the Delphi method have investigated how colour schemes can mitigate stress in confined hygiene areas within space habitats. Additionally, short-duration simulated spaces have been used to test multicoloured lighting environments and explore their psychological effects on participants, aiming to enhance well-being and performance. Other approaches have directly examined visual perception in microgravity through parabolic flight experiments, which simulate weightlessness for brief periods, or through postural techniques that emulate gravitational unloading. These methodologies provide insight into the human visual system's adaptability but are limited in their duration compared to long-term missions. Although no direct empirical data currently confirm changes in colour perception during extended space missions, ophthalmological studies have documented physiological alterations due to microgravity, including optic disc oedema, globe flattening, choroidal folds, and hyperopic shifts. These effects suggest possible impacts on retinal and cortical processing, yet astronauts have not self-reported changes in colour perception. This discrepancy raises questions about compensatory mechanisms, such as chromatic constancy and contrast adaptation, which might preserve perceptual stability despite physiological changes. Furthermore, distinguishing the intent of colour sensitivity assessments is critical. The context in which colour perception is evaluated, such as the observed environment, can trigger compensatory cognitive or physiological mechanisms that play a vital role in interpreting the results. Given the absence of evidence indicating altered colour sensitivity in space, it is plausible that the physiological mechanisms underlying circadian entrainment via light at specific wavelengths remain functional in microgravity. This hypothesis supports the continued application of light-based interventions for circadian regulation in extraterrestrial settings. However, targeted research is essential to verify this assumption and optimise lighting design for future long-duration missions.