Apple added a hibernate feature called Safe Sleep to their notebook starting with the PowerBook G4 (Double-Layer SD). Safe Sleep saves the notebook's state to disk and is handy if you run completely out of juice while sleeping with unsaved documents or windows open.
It's that dimmed, grey screen that you may have seen after plugging in your machine after an extended MacBook nappy session:Hibernate/Safe Sleep an undoubted lifesaver – unless, of course, you have a MacBook Air and can't afford the 2GB of disk space that it requires. The default behavior on all MacBooks is to sleep and hibernate but that can take a long time while the contents of the memory are saved to disk.
Enter SmartSleep, a freeware preference pane that dynamically sets the sleep state of your machine. SmartSleep lets your notebook sleep (without hibernating) when the battery is above a user-defined level. If your battery level drops below that threshold (the default is less then 20 percent or 20 minutes) it will switch to sleep and hibernate mode. So, it's essentially the best of both worlds.