And once sleep mode is activated, all your active apps will stop working. If you have a background app running with the App Nap feature enabled, it won’t be able to prevent your Mac from going into sleep mode.
Eventually, the entire Mac goes into sleep mode depending on your configured power settings. The screen of your Mac automatically turns off after a certain duration of inactivity. And if that app isn’t currently doing something for you - playing music, downloading a file or checking email, for example - App Nap conserves valuable battery life by slowing the app down.”
“… OS X can tell when an app is completely hidden behind other windows.
