How did Muslims in the past wake up for fajr?

I hope this question is relevant enough for this section of StackExchange, as it's more towards history rather than Islam itself. Nowadays, people depend on alarm clocks or the loud sound of athan coming in from the window (in which case, the muathin probably also woke up from an alarm clock) in order to wake up for fajr, in which the time may shift between 4am and 6am (in my country at least). If someone were to ask me to wake up at those times without an alarm clock, I would find it completely impossible. My question is, in the past, before the invention of electricity and alarm clocks, how did Muslims wake up reliably for fajr?