I did not know that yesterday!

Why is the snooze function on alarm clocks always set to nine-minute intervals?

Before researching the answer, I would have thought it's because nine
minutes is the approximate length of time it takes to fall back asleep
before being roused by the piercing beep of the infernal alarm clock.

But as it turns out, it's really all a matter of mechanics. Back in
1956 when the snooze button was first introduced, alarm clocks had
standardized gears. The snooze gear had to mesh with the teeth of the
other gears. Due to the configuration of the gears, a nice, round
10-minute snooze cycle was out of the question, so the engineers had
to choose between nine minutes or 10-plus minutes. As we all know,
punctuality is a virtue, so the engineers went with nine minutes.

Various attempts have been made to change the nine-minute snooze cycle
-- manufacturers have tried five, seven, and ten minutes, but a
nine-minute snooze has become the unofficial standard.