Knocker-ups were a thriving profession that started during the Industrial Revolution. They woke up their clients to make sure they got to work on time. Either with a long stick, to knock on windows even on the higher floors, or (in at least one case) with a pea shooter. The knocker-up wouldn’t leave until they knew their client was awake. The job lasted until the 1920s, when alarm clocks became both reliable and affordable.

(Source: Wikipedia)