Introduced in 2005's PowerBooks, 'safe sleep' (or 'hibernate mode' as it's known in the Windows world) is a feature designed to preserve the current state of your Mac when you put it to sleep. Enabled by default on Apple's notebooks, the feature writes the entire contents of your RAM onto the hard disk when you put your Mac to sleep. The practical upshot of this is if your Mac loses power, the next time you start it up everything should be restored to exactly the way it was when you put your Mac to
