If your computer goes to sleep, the backup cannot take place. Set your computer for higher performance and lower power conservation setting. You can compromise by allowing the display to go to sleep.
For a Mac
Go to System Preferences, then Energy Saver
- Set Computer Sleep to “Never.” Allowing the display to sleep is okay.
- Leave your computer running overnight (open and running if a laptop).
- Display sleep can be set to any time you would like.
Or your Energy Saver might look like the image below. Check the box that says "Prevent computer from sleeping automatically when the display is off."
Go to Control Panel > Power Options. Then make sure it is set to "High Performance" and that the computer is not set to sleep. You can still get power savings by allowing the display to go to sleep.