Converging indicators for assessing individual differences in adaptation to extreme environments.

It is well known that microgravity results in various physiological alterations, for example, head-ward fluid shifts which can impede physiological adaptation. Other factors that may affect crew operational efficiency include disruption of sleep-wake cycles, high workload, isolation, confinement, stress, and fatigue. From an operational perspective, it is difficult to predict which individuals will be most or least affected in this unique environment given that most astronauts are first-time flyers.