What is Power Nap?
Power Nap, available on Mac computers with flash memory, lets some Mac computers stay up to date even while they’re sleeping. When the Mac goes to sleep, Power Nap activates periodically to update information. The information that’s updated depends on whether your Mac is running on battery power or plugged into a power adapter.
When your Mac is asleep and using battery power, Power Nap:
Checks for new messages in Mail
Updates events in Calendar
Updates other iCloud events
When your Mac is plugged into a power adapter, Power Nap can also do Time Machine backups.