The first American alarm clock could only ring at 4 o'clock in the morning.
Levi Hutchins of New Hampshire built it in 1787 so that he would never oversleep, and he only ever needed one wake-up time. The mechanism had no way to set a different hour.
Why is this true?
Adjustable mechanical alarm clocks were patented in France in 1847 and became common by the late 1800s. Before that, factory workers in Britain paid knocker-uppers to tap on their windows with long poles.
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Most inventions arrive by accident, in the wrong order, or decades before anyone finds a use for them. Microwaves, sticky notes, safety glass and vulcanised rubber all started as mistakes or side effects. The facts here trace how the thing actually came about, note who noticed the accident and why it mattered, and point at a source so you can follow the full story if it hooks you.