Knocker-ups woke people before alarm clocks were common

​From the industrial revolution until the 1920s, British people paid "knocker ups" to wake them. These were a common sight in places where people worked in shifts or at unusual times: when the own sense of time was overstrained to rouse by themselves. 

When the profession just began to evolve, knocker-ups used to rap or ring at the doors of customers. But neighbors complained – they were aroused unwantedly!  
 
To avoid this kind of disturbance, knocker ups then started using a long stick to tap on their clients' bedroom windows: loudly enough to wake those who paid them, but silently enough for the people living nearby to stay asleep. 
 
An interview with a female knocker up appeared in The Huron Expositor May 22, 1978. She tells she earned 30 to 35 shillings a week by serving around 30 houses or more a day.  
 
As Mrs. Waters explains, knocker ups were also employed by people who did own an alarm clock – but just like today, people got used to the sound of it and either slept through it or just switched it off and continued sleeping.