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I can’t tell you how many messages I get from readers whose Macs refuse to get a proper night’s sleep—they refuse to go to sleep, sleep, but then wake almost immediately; or fail to awaken when stirred. I can, however, attempt to provide you with the most common reasons why Macs sleep poorly. They include:

System Preference settings Before you rip out your hair, wander over to the Energy Saver system preference and make sure that, in the Sleep tab, the Put the Computer to Sleep When it is Inactive For slider is not set to Never. Then open the Classic system preference and check the Classic Sleep settings in the Advanced tab. Again, you want that slider set to something other than Never if you expect your Mac to go to sleep automatically while running Classic applications.

If your Mac is configured to receive faxes and you don’t use it to receive faxes, open the Print & Fax system preference, click the Faxing tab, and disable the Receive Faxes on This Computer option. Your computer could be keeping a wakeful eye out for incoming faxes.

If Internet Sharing is switched on, a Mac running OS X 10.4 won’t sleep.

Corrupt files If certain preference files become corrupted, your Mac could refuse to sleep properly. Turn off Bluetooth in its system preference, quit system preferences, and give its preference file the boot—found at yourusername /Library/Preferences/

A corrupt PowerManagement preference file can also be a problem. Quit system preferences and toss these files:

/Library/Preferences/SystemConfiguration/ /Library/Preferences/SystemConfiguration/

Enter your administrator’s password when requested, restart your Mac, and reconfigure Energy Saver.

Peripherals Some printers, scanners, and Bluetooth devices (mice, in particular) can keep your Mac from going to sleep. Shut down your Mac and unplug everything but the monitor, keyboard, and mouse (surely you’ve got your old wired input devices around somewhere). Restart and see if your Mac behaves itself. If so, add one device at a time, restart, and see how the Mac fares. When the problem recurs, cock a suspicious eye at the device you last added. Look for updated drivers for that device.

If a Bluetooth device appears to be the problem, launch System Preferences, click Bluetooth, and switch off the Allow Bluetooth Devices to Wake This Computer option. (Note: If a Bluetooth device isn’t the problem, you may create a new problem by turning off this option as you need it on for your Bluetooth mouse or keyboard to wake a sleeping Mac.)

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