No one knows for sure, but the most likely theory is it was invented that way.
The snooze was first introduced with a purely mechanical clock. It was thought that the snooze period should last no longer than 10 minutes. Not having given more specific instructions to the clock maker, it was decided that the specific length was not really all that important. The part of the clock that regulated the snooze time was made to function without precision. The actual snooze time could range anywhere between nine and 10 minutes, and it changed each time it was activated.
This theory was later verified with an old electro-mechanical clock made by GE. Its snooze time ranged from nine minutes to nine minutes 30 seconds simply because the mechanism was not made to be precise.
The theory goes on to say that when digital clocks were first made, the manufacturers were just as baffled as to why snooze lasted nine minutes as most of us are today. They just figured there was some important reason behind it, so they continued the tradition.
Other theories include:
- It has to do with the power cycles of AC current;
- A nine-minute snooze allows for six activations in a 60 minute period, allowing one minute for time to lapse between each activation;
- Nine minutes sounds better than 10 minutes; and
- People fall into a deeper sleep after nine minutes.