How did Muslims wake up for Fajr prayer in the absence of modern alarm clocks?

One who is accustomed to waking up at a specific time, or resolves to wake up, will often wake up without an alarm, as we see today. Allah has made the rooster crow at specific times of the night, and the Prophet, peace and blessings be upon him, would rise for his prayer when he heard the sound of the rooster. Some scholars have issued fatwas permitting reliance on a proven rooster to know prayer times. And if the Companions feared they would not wake up for prayer, they would appoint someone to stay awake to wake them, as the Prophet, peace and blessings be upon him, did with Bilal on one of his journeys.