When your computer is put to sleep or hibernates, the Backup Agent service is paused and cannot run backups during that time. This will either prevent scheduled backups from running in the first place or it will pause backups which are in progress; backups paused may not resume seamlessly. In order to ensure backup jobs complete successfully, you must have the computer kept on, awake, and connected to the internet.
Here are the relevant articles for editing your computer's power settings: