A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

# Why does the alarm clock snooze button give you nine extra minutes, not ten?

November 26, 1999

Dear Cecil:

When my roommate's alarm goes off, he invariably presses the snooze bar. This continues in nine-minute cycles until I have to rouse him myself. All the alarms I have seen have a nine-minute snooze interval. Is this a standard number, and if so, where did it come from?

Cecil replies:

What a relief to quit dealing with the federal government and get back to the truly gut questions of our time. Although I gotta tell you, dealing with the feds was a piece of cake compared to this one. We consulted with numerous clock manufacturers, clock engineers, and clock buffs and amassed the following theories:

(1) Focus groups found that people preferred a snooze delay of eight to twelve minutes. OK, but then why not a ten-minute interval?

(2) Engineers believe their bosses come to check on them every ten minutes. Ho ho!

(3) Physiologists have found that a sleeper who doesn't want to get up will fall back into a deep sleep if left for longer than nine minutes. Yeah, right.

(4) Five minutes seems too short and ten minutes seems too long. Nine minutes may seem better than ten while not being significantly different. My reaction: Bah. Nine minutes does not seem better; it seems stupid.

(5) On LED (the old red display) clocks, the snooze function will work for only 60 minutes, so you want to fit the greatest possible number of snooze periods into that time. Nine minutes gives you six snooze periods with a minute's leeway each time for pressing the snooze bar. "Nonsense," one engineer commented. No argument here.

(6) "I figured it was actually 512 seconds (2^9)," one informant speculated. "Or maybe, since the clock is counting (typically) the power cycles from the wall socket, it's because nine minutes is 32,400 cycles, very close to 2^15 (32,768)." Engineer's comment: Nice try, bub, but clocks don't count that way.

(7) General Instruments, one of the first designers of the chip used in LED clocks in the late 60s, set the chip logic to allow a nine-minute delay. Others continue to use this chip or copied the idea without changing the interval (e.g., National Semiconductor's type MM5370 digital alarm-clock chip — I tell ya, do we research this stuff or what?). Fine, but why nine?

(8) On a digital clock, nine is the greatest interval obtainable by advancing some sort of "snooze counter" on the ones column. But why mess with the ones column? Why not put the snooze counter on the tens column and advance that by one?

(9) In the days of dial clocks, the snooze interval was originally intended to be ten minutes max, but precision was unimportant and engineers were content if they could make the interval nine minutes and change. When the industry switched to digital, clock designers figured the standard snooze interval was nine minutes; "and change" went out the window.

Now we're getting somewhere. Partial confirmation of this view comes from Jay "Pappy" Kennan, a clock collector who took apart an old GE electromechanical clock with one of the earliest snooze buttons. (Pappy helpfully posted photos of the clock's innards on his Web site; see the links at the bottom of www.ma.ultranet.com/~jayman. [Link now defunct]) The clock's snooze-gear mechanism was not precise; the snooze interval could be anywhere from nine to nine and a half minutes. Pappy's opinion, seconded by a clock engineer, was that the original, none-too-ambitious designers wanted a clock with a snooze interval in the nine-to-ten-minute range.

So what may have happened was, some early chip designer inspected an old mechanical clock with a snooze button, figured that a nine-minute snooze interval had been ordained by the clock gods, and built it into his chip — and we've been stuck with it ever since. That's my theory, and I'm sticking to it.

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