That's only a data point, but the Canadian newspaper The Huron Expositor published on 22 March 1878 a lengthy interview with a Mrs Waters, a 70-year-old widow in the "town of B___, North England", who told the reporter about her daily life as a "knocker-up" (she was retired now), and how she had been led to take up this job after her husband had become disabled. She claimed that she was among the first to do so and that she was knocker-up for thirty-five years - so she would have started in the 1830s if we follow her timeline -, "never [earning] less than thirty shillings a week".
She briefly talks about the waking up part:
When did I get my sleep, do you ask? I'll tell you. I always went to bed at nine o'clock every night, except Saturday night; and having a tired body and a contented mind, I was not long in dropping asleep. And I was up again at half-past two to the minute; for my first customer lived a good 20 minutes'·walk from my house, and you know he had to be awakened at three o'clock. Well, for some time I had no one else to arouse until four o'clock, so I generally came home. Before I went out in winter I got a cup of tea, so I kept the fire in ; but in summer I let it I go out, and did not care to light it again until I came back from the early customer. Then I always made my poor husband a cup of tea, after which he slept better than in the fore part of the night. You see he had to awaken me; for being young and very active during the day, I slept soundly. But what between him and the alarum, I never over slept myself; no, not even once. But I after I had been about six or seven years at the job, I got to awaken quite naturally like. It was well I did; for when my husband died, I had no longer him to depend on.
So, in a nutshell: she went to bed early (she did not sleep during the day) and usually woke up by herself out of habit after doing so for several years, or her husband did wake her when he was still alive, or the alarm-clock ("alarum") did.
The "alarm" part is intriguing, because the popular narrative is that knocker-ups were needed because "alarm clocks were neither cheap nor reliable". However, here is what Mrs Waters says:
The reason why knocking-up is so widespread nowadays is this: people soon get so used to the alarum-clock that if fails to awake them, or if it awakes them, they are at times so sleepy that they drop off again before the alarum runs out. This was the case with the person who asked me to awaken him ; he had lost many mornings through over-sleeping the time.
It turns out that alarm-clocks were in fact widespread and relatively cheap, sold for the equivalent of a few days of wages at worst (please correct me if I'm wrong here). This advertisement (Watchs! Watchs! Watchs! Clocks! Clock! Clocks!) in a Welsh newspaper from 1864 lists prices starting at 3 shillings and 9 pence, "warranted to awaken the soundest sleeper at any hour". In France, alarm-clocks (réveil matin) were also sold for as low as 5 francs in 1877 (Chambon ad). These cheap clocks were not of high quality: Larousse's Grand dictionnaire universel of 1875 calls them "clocks of the most ordinary type, not requiring much precision", and even "coarse clocks". If we believe Mrs Waters, those exhausted workers would sleep through the clock anyway, so they would require a knocker-up person to wake them up. Mrs Waters, the professional knocker-up, did not.
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