Physiological Effects

The heart and blood vessels adapt to weightlessness, and the heart can weaken without the constant work of pumping against gravity. Crews often experience orthostatic intolerance — dizziness on standing — when they return to gravity.

On the ISS the crew sees sixteen sunrises a day, and mission schedules can shift sleep times; the resulting disruption of the body clock degrades sleep, alertness, and performance.

Spaceflight alters the immune system — some functions are dampened while latent viruses can reactivate — a concern for crew health on long missions far from medical care.

Long missions in a confined habitat, far from family and unable to leave, place real psychological demands on crews — from mood and interpersonal tension to the cognitive effects of monotony — that grow with distance and duration.

In the first days of flight, the conflict between the eyes and the balance organs of the inner ear causes disorientation and nausea as the brain adapts to weightlessness; a mirror-image readaptation occurs on return to gravity.