"Sonic Heroes" was developed by the 19-member Sonic Team USA in San Francisco to commemorate the "Sonic" series' 12th anniversary. and lasted 20 months. The majority of the development team had worked on previous "Sonic" games. Iizuka did not want to make a sequel to "Sonic Adventure 2" (2001), as he worried it would only appeal to "Sonic" fans. Interested in returning to gameplay similar to the Genesis games, Sonic Team decided to design "Sonic Heroes" so casual players not familiar with "Sonic" could adapt. Iizuka said the development period of "Sonic Heroes" was the most stressful of his career because of deadlines and Sega management. A fellow designer became ill, so he worked relentlessly, lost 22 pounds, and suffered from insomnia.