Why is early Adhan specific to the Fajr prayer?

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The early Adhan is exclusive to Fajr because it is the time of sleep, thus serving the purpose of waking people so they may prepare and proceed to the prayer. This necessity does not exist for other prayers. It is narrated that the Prophet, peace be upon him, said: "Indeed, Bilal calls Adhan at night so that your sleepers may be awakened and those who are praying may return [to their rest]."