A viewgraph presentation by Jennifer Fogarty, Ph.D., NASA JSC
Space Adaptation Syndrome (SAS): "Approximately 70% of all astronauts traveling into space experience SAS symptoms, which range from nausea and light headedness to vomiting. The exact cause of SAS is unknown, but it is believed that it is caused by conflicting sensory inputs from the vestibular organ (inner ear) and the eyes. SAS symptoms normally last only for a day or two and can be treated by medication.
View the pdf file HERE.