Mary Smith using peas as an alarm clock in London’s East End

The modern worker rolls out of bed, groans, and turns off an alarm clock. But industrial-era British and Irish workers relied on a different method for rising each morning. In the 19th century and well into the 20th, a human alarm clock known as a “knocker-up” (knocker-upper) would trawl the streets and wake paying customers in time for work. Armed with sticks—or, in the case of Mary Smith, a pea shooter—they tapped on windows or blasted them with dried peas.

During the Industrial Age, people toiled at unusual hours in mines or factories. They could have used alarm clocks—adjustable versions had been invented by the mid-19th century. But they were still relatively expensive items, and unreliable ones, at that.

Whether they wielded rods or pea shooters, knocker-ups became familiar presences throughout the United Kingdom. Many of them…