Safe Sleep ensures that data stored in main memory will not be lost should the computer shut down due to the battery being drained completely. Prior to your computer entering sleep, Safe Sleep automatically saves the contents of main memory—such as desktop settings, open applications, and other work in progress—to the hard drive.
There are two situations in which a computer will enter safe sleep:
- In the event the battery becomes completely depleted while the computer is asleep, the computer will shut down. But when a power adapter is connected or a freshly charged battery is installed, the computer can be restarted. Press the power button as if turning the computer on and it will automatically return to the desktop state that existed prior to entering sleep. This means that applications and files will remain exactly as they were prior to the computer being put to sleep, making it easy for you to continue with your work.
- With the release of the OS X Mountain Lion v10.8.2 supplemental update 2.0, a new feature was introduced to enter safe sleep after four hours of the computer being connected to AC power. This is an effort to comply with the European Energy Standards (ErP Lot6). This will only occur if there is no wireless or Ethernet activity and no activity from external devices such as USB storage devices.
This is normal behavior for the following models:
- MacBook Pro (Mid 2012 and later)
- MacBook Pro (Retina, Mid 2012 and later)
- MacBook Air (Mid 2012 and later)
- iMac (Late 2012 and later)
- Mac mini (Late 2012 and later)
(Source: http://support.apple.com/kb/HT1757 from 9 October 2013)