Space adaptation syndrome (SAS) is a condition experienced by astronauts and cosmonauts during their first few days in space. It is caused by the lack of gravity and the associated sensory changes. Symptoms include nausea, vomiting, dizziness, headaches, fatigue, and disorientation. Astronauts may also experience difficulty sleeping and concentrating due to the unfamiliar environment. SAS usually resolves itself within a few days as astronauts adjust to their new environment.