How did Natan survive nine years in a Siberian Labor Camp/Prison?

From a very young age, Natan was a chess prodigy. Growing up in the USSR, he won prize after prize for his chess skills. He would constantly compete, and constantly win. It has been said, that in order to keep his mind busy during those horrific nine years, Natan would play chess against himself. This was in order to keep his brain engaged. After being released from prison and arriving in Israel, Sharansky continued to compete for fun. He even was part of a competition in Israel where he beat one of the top world-ranked players.