IMac wakes itself up, then puts itself back to sleep

My iMac constantly wakes itself from sleep. The problem seems to have started around the time that I upgraded to Yosemite. If I leave the computer in sleep mode overnight, it will happen dozens of times throughout the night. The computer wakes up, but the monitor never comes on. I hear the hard drive spin up, I see my mouse’s LED turn on, then after a few seconds it goes back to sleep.
I have tried everything in this KB article (https://support.apple.com/en-gb/TS5357) with no luck. I have done SMC and PRAM resets with no luck. I’ve tried unplugging all USB devices prior to sleep in case one of them was waking the machine, with no luck. I have no Bluetooth devices, and Bluetooth is turned off. “Wake for network access” is turned off. I have no scheduled power or wake/sleep events. I have no file or device sharing enabled.
If I do a “wake reason” terminal command, I have page after page of the following message with varying timestamps: “Mar 28 11:45:20 iMac kernel[0] <Notice>: Wake reason: RTC (Alarm)” The info I’ve found about this says, “Real Time Clock Alarm, is generally from wake-on-demand services like when you schedule sleep and wake on a Mac via the Energy Saver control panel. It can also be from launchd setting, user applications, backups, and other scheduled events.” Again, I have no scheduled wake settings, “wake for network access” it off, and I have no applications that should be waking the computer.
I’m at my wits’ end with this problem. At this point, the only thing I can think to do is a full reinstallation of OS X, but I’d really rather not have to go that far. Any suggestions?
Specs:
iMac (20-inch, Mid 2007)
OS X 10.10.2
Processor: 2.4 GHz Intel Core 2 Duo
Memory: 4 GB 667 MHz DDR2 SDRAM
Graphics: ATI Radeon HD 2600 Pro 256 MB

Please read this whole message before doing anything.
This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.
The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.
Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.
The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem?
After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.

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IMac wakes itself up, then puts itself back to sleep

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