How Do I Use Time Machine In Sleep Mode?

Time Machine does it backup every hour, day, week etc but if the backup is up to date, example only 1 minute ago and I create a document and save it to the desktop or a folder, then put the Mac in sleep mode and not return to the computer for at least 2 hours will Time Machine backup the document/ folder in about 1 hour’s time. Or does it commence the backup only when I wake up the computer?
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Danny