Why do snooze buttons only work for 9 minutes?

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I could be wrong, but I get the feeling the person who posted this question means why was 9 minutes chosen for the intervals as opposed to maybe 5 or 10 minutes. Nine minutes does seem like an arbitrarily chosen unit of time.

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Well, I have researched this topic and yielded some theories. If the snooze button is 9 minutes then the digital clock would only have to keep track of the last digit, because the last digit would go down by 1.

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By the time the snooze feature was added in the 1950’s, the innards of alarm clocks had long been standardized. This meant that the teeth on the snooze gear had to mesh with the existing gear configuration, leaving engineers with a single choice: They could set the snooze for either a little more than nine minutes, or a little more than 10 minutes. But because reports indicated that 10 minutes was too long, allowing people to fall back into a “deep” sleep, clock makers decided on the nine-minute gear, believing people would wake up easier and happier after a shorter snooze. We’d tend to disagree with that logic, but, then, we must be in the lazy minority. Although today’s digital clocks can be programmed to have a snooze of any length, most stick with nine minutes because that’s what consumers expect.

http://www.mentalfloss.com/trivia/questions/?m=200609

First answer by ID0417413382. Last edit by Exivion. Contributor trust: 63 [recommend contributor]. Question popularity: 134 [recommend question]