system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did people used to wake up early before alarm clocks?
gpt
1) **Drinking water** - It used to be a common plan that when someone needed to wake up before sunrise they would just drink a great deal of water.  Full bladder would mean you'd wake up having to urinate.  History is full of pre-dawn raids and military attacks that used this method.  
2) **Clepsydra** - Water clocks have been used for thousands of years.  They weren't clocks so much as timers.  You'd have a container that would leak a small amount of water, and then a floating bob would strike an alarm when the water lowered to a certain point.  
3) **Knocker-Up** You could pay someone to stay up all night and then wake you in the morning.  Large factories often employed Knocker Ups for their employees.  And you could be guaranteed they wouldn't stop knocking until you were up.