It is mainly caused by the temporary insufficient blood supply of the cerebrum. When one crouches, his lower limbs buck and the blood vessels of the lower limbs is pressed so that the blood cannot flow to the lower limb, causing the shortage of blood of the lower limb. When you stand up suddenly, the blood vessels of the lower limb is unblocked again and a large quantity of blood moves downward, causing the inadequate blood flow of the upper part of the body. Without abundant oxygen supply, the brain and eyes break down, causing dizziness and stars before eyes. If one is in poor health, the situation will be worse. However, when such situation happens, do not be panic and there is no need to go to hospital. When the head cannot supply sufficient blood, the heart will speed up work and deliver blood upward, so the body will recover soon.