In the later part of his life, George had recurrent, and eventually permanent, mental illness. Although it has since been suggested that he had bipolar disorder or the blood disease porphyria, the cause of his illness remains unknown. After a final relapse in 1810, a regency was established. His eldest son, George, Prince of Wales, ruled as Prince Regent until his father's death, when he succeeded as George IV. Historical analysis of George III's life has gone through a "kaleidoscope of changing views" that have depended heavily on the prejudices of his biographers and the sources available to them.