edit: It sounds like the first alarm clock with a snooze might have been synchronous so maybe it's worth thinking more about that :)

http://en.wikipedia.org/wiki/Telechron http://blog.onlineclock.net/evil-snooze-clock-history/

It's a bummer the Wikipedia article for Alarm clocks cites the straight dope article which links to some suggestive information that is totally missing now. It's too bad that Wikipedia doesn't store source information that is important for verifying the statements in articles in case the linked source disappears (or does it?)

originally, and probably wrong:

Yeah, some clocks used this, but I don't think it was most clocks and unless the first clock with a snooze functionality was this type, it doesn't give any more insight into why 9 minutes was chosen.