If the sync schedule of a sync item is set to a time when the Mac is asleep, macOS automatically delays the sync until the Mac wakes up again.
There might be instances where you want a sync to run while you’re not using your Mac, such as if a sync is expected to copy many files that should be available on the next morning. In this case, the only option is to keep your Mac awake until the sync has finished.
In the System Settings, under Lock Screen, you can control when the Screen Saver starts and when the display is turned off after a period of inactivity.
When the Screen Saver starts, the Mac is still awake and syncs can still run. When the display is turned off, the Mac might still remain awake for some time, but can go to sleep as soon as no process prevents it from doing so. To actively prevent automatic sleep when the display is turned off and the Mac is connected to the power adapter, in the System Settings, under Displays, click on Advanced (at the bottom) and enable the option “Prevent automatic sleeping on power adapter when the display is off”.
