What is Safe Sleep?
With the release of the first Intel Mac laptops, Apple introduced a new type of sleep state known as Safe Sleep.
Safe Sleep stores the contents of RAM and other system settings onto the hard drive itself so that if the laptop looses all power, your work is not lost and you can recover quickly once power is restored.
Traditionally, when a computer sleeps (and is unplugged), the battery provides enough power to keep the laptop on and the contents of the RAM in tact. If you were to fully drain the power or remove the battery, the system would shut off and any unsaved work would be lost. On PowerBook G4 models, there was a backup battery that allowed you to hot swap a battery or drain the main battery without losing any data.
Safe Sleep takes this one step further. When you put any MacBook to sleep (close the lid or select Sleep) the computer first writes the contents of the RAM and other settings to the hard drive.
The battery or ac adapter still keeps the laptop on (hibernated) and keeps the RAM contents in tact which allows for the quick wake up normally associated with waking a computer from sleep. You can press any keyboard key or open the lid to wake the laptop normally.
If the battery fully drains or is removed, Safe Sleep is used to “wake” the computer by accessing the files that were stored by Safe Sleep on the hard drive.
Note: Since the computer is technically off, the keyboard will not wake a computer in Safe Sleep and the power button must be used. The computer will boot to a dimmed screen with a progress bar on the button as it loads the saved contents on the hard drive.
Safe Sleep is found on all MacBooks and is enabled automatically. It really just adds another safeguard to saving your work (and time) when the battery drains or you lose power. It works in conjunction with traditional sleep methods which allow for a faster wake time.
