Macintosh computers are designed so that when you’ve finished working with them temporarily, you can put them to sleep. This puts your Mac into a low-power consumption mode rather than shutting it down completely. There are several advantages to this:
You don’t need to quit your applications, though you should save your files in case you have a problem such as a power cut.
You don’t need to restart your computer and relaunch your applications the next time you want to use it.
Your Mac starts up very quickly when you reawaken it; this takes just seconds.
So, if you have finished working but plan to come back to your Mac soon, click the Apple Menu and select Sleep.
Your Mac will go to sleep immediately. Some Mac models have a light on their power button (it’s on the front of some portables) that fades on and off, looking like your Mac is breathing peacefully.
You can also put your Mac to sleep by pressing its Power button. On some models, such as iMacs, this puts the Mac to sleep immediately; on others you must click Sleep in the dialog that displays, or press the S key on your keyboard.
To wake your Mac, just press any key on the keyboard. It takes a few seconds, and you’ll see your screen light up as your Mac awakens, exactly in the same state as it was when you put it to sleep.