What is Power Nap on Mac?
Power Nap, available on Mac computers with flash memory, lets some Mac computers stay up to date even while they’re sleeping. When your 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 (a portable Mac) or plugged into a power adapter (a portable Mac or desktop Mac).
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 activities such as downloading software updates and performing Time Machine backups.