For Mac computers that are started from an mechanical hard drive, OS X includes a deep sleep mode known as safe sleep. Your Mac may enter safe sleep if your battery begins to run low, or your computer is left idle for a long time. Safe sleep copies the contents of memory to your startup drive, and powers down the computer. This helps you when your battery is depleted on Mac notebooks. All you have to do is connect your notebook to AC power again to pick up where you left off, without losing your work.
To wake your Mac from safe sleep, press its power button. If you are using a Mac notebook and your battery is low, connect the AC adapter first. When waking from safe sleep,
a progress indicator appears indicating the previously stored contents of memory are being read from the startup disk, and copied back into RAM.