Backups & restore points · Speed, progress and the first backup
Will my Mac go to sleep in the middle of a backup?
Short answer: Plugged in, a backup keeps your Mac awake until it's done. On battery only Back up now does (above 30%); automatic backups let the Mac sleep and resume later.
Plugged in (or on a desktop Mac), a running backup keeps your Mac from dozing off through inactivity until it's done.
On battery, only a backup you start with Back up now does — and only while the battery is above 30%. Automatic and scheduled backups let your Mac sleep as it normally would, and simply carry on from where they stopped next time, so nothing is lost. That's why Back up now is the large button on every backup card.
Closing a MacBook's lid on battery still puts it to sleep; no app can prevent that.