I always wondered this, and here is the answer:
mentalfloss.com
By the time the snooze feature was added in the 1950s, 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 early 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.
The more you know....
Why Does the Snooze Button Only Give You 9 More Minutes of Sleep?
Why not make it an even 10?
mentalfloss.com
By the time the snooze feature was added in the 1950s, 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 early 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.
The more you know....
