The Soviet Union has attempted to alter the circadian rhythms of their cosmonauts by having them awaken 20 minutes earlier on each day during a long-term flight, but such efforts have not proved successful and are no longer tried. The human body can withstand a much higher g loading in the former mode. The g loads themselves, which have varied from 3 to 8.2 times the gravity experienced at the Earth's surface, have created no problems. In the U. S. Space Shuttle, however, crews take a reentry decelerative force of about 1.5 g in feet-first position, which causes blood to push toward the feet. In order to prevent the pooling of blood, Shuttle crews are therefore provided with suits that produce pressure on the lower part of the body.
Physiological changes start to occur from the first moment of achieving orbit.
Body fluids redistribute themselves toward the head. As they do, the face puffs out, and some astronauts experience sensations similar to those of a head cold. The curve of the spine also straightens out somewhat, so that persons may gain an inch or so in height while in a weightless environment. Many astronauts have experienced a mild lower backache as a result of the different stresses being placed on the back muscles in space. The vestibular system consists of the otolith and the semicircular canals of the inner ear which constitute the body's organs of balance.