Low End Mac’s Safe Sleep FAQ

Apple introduced a new feature, Safe Sleep, with Mac OS X 10.3 Panther in 2003. When enabled, Safe Sleep writes the contents of your Mac’s memory to a file named sleepimage on its hard drive before putting the computer to sleep (this works like the Hibernate command in Windows). In case the Mac loses power while sleeping, it can restore the state of memory using this file – complete with open documents. This is a lot faster and more efficient that restarting the computer, relaunching your applications, and resuming where you left off with your documents and Web browsing.

It generally takes about 20 seconds for your Mac to write the sleepimage file to its hard drive, a bit less to SSD. If your Mac is writing the sleepimage file to a hard drive, it should not be moved during this time. Once the file is written, your Mac will enter sleep mode.

Safe Sleep is on by default for MacBooks and Intel-based Mac minis; it is off by default for PowerPC Macs.

Which Macs Support Safe Sleep?

Most Macs introduced since Sept. 2003 support Safe Sleep. It has been reported to work with “the majority of G4s” including the following Macs:

  • iBook G4, all models
  • aluminum PowerBook G4, 1 GHz and faster models (reports of problems with the fan not running on the 12″ 867 MHz PowerBook)
  • eMac, 1 GHz
  • Mac mini
  • some G4 Power Macs

It seems reasonable that Power Mac G5 and iMac G5 models would support Safe Sleep, but the only discussion of it I’ve found reports 100% failure to enter Safe Sleep mode on G5 Macs.

Apple’s list of models that support Safe Sleep is much shorter. To the best of our knowledge, every Intel Mac supports Safe Sleep.

Why Would I Want to Use Safe Sleep on a Desktop Mac?

Just as a notebook wakes up faster and resumes its state after losing power if Safe Sleep is enabled, a desktop Mac will do the same thing. This can be especially helpful if you’re moving a Mac mini or Safe Sleep supported iMac from one location to another.

Why Would I Want to Disable Safe Sleep?

If you want to be able to Sleep your Mac and move it right away, disable Safe Sleep. If Safe Sleep is enabled and you move your Mac, there is potential for damaging the hard drive while it is writing the sleepimage file. (This is not an issue with SSD.)

How Much Hard Drive Space Does Safe Sleep Use?

The sleepimage file will take exactly as much drive space as the amount of physical RAM installed in your Mac. If your hard drive does not have sufficient free space for the sleepimage file, it will not create it. The file will be deleted when you shut down or restart your Mac.

Do I Have to Enable Safe Sleep?

Intel-based Mac notebooks (MacBooks) and Mac minis have Safe Sleep enabled by default. iBooks, PowerBooks, and Intel-based MacBooks produced prior to the introduction of Safe Sleep have it disabled by default.

How Do I Disable/Enable Safe Sleep Mode?

There are Terminal commands for turning Safe Sleep on and off, but there’s an easier way. The shareware SmartSleep preference pane (OS X 10.6 Snow Leopard and higher, so Intel only) and the trialware DeepSleep Widget let you choose one of three sleep options:

  1. Sleep without Safe Sleep.
  2. Safe Sleep.
  3. Safe Sleep and power off computer.

According to the SmartSleep page, some Mac OS X 10.4 Tiger users find that it doesn’t work, and some users report that the Sleep command no longer functions when SmartSleep is installed. The page includes full instructions for removing SmartSleep if that is the case.

Another option is the DeepSleep widget, which supports Mac OS X 10.4.3 and later and offers five different options.

The Terminal Commands

These commands should work in all versions of Mac OS X. Enter the Terminal and type (or cut and paste):

  • sudo pmset -a hibernatemode 0 to disable Safe Sleep. Then sudo nvram “use-nvramrc?”=false only if you are using a PPC Mac. Finally, sudo rm /var/vm/sleepimage to delete the sleepimage file. Now restart your Mac.
  • sudo pmset -a hibernatemode 3 to resume using Safe Sleep. Then sudo nvram “use-nvramrc?”=true only if you are using a PPC Mac.

Further Reading

This article was originally published in June 2009 and has since been updated.

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