In deep sleep, the mind is shut down, and thinking is not possible. When reflecting on deep sleep, people say 'I slept happily,' indicating the presence of a common eye that transcends waking, dreaming, and deep sleep. This common eye points to the real self, the silence of the universe. By investigating and following this pointer, one can realize their true self, which is the quiescence and silence of the universe according to Advaita Vedanta.