When you wake up `at’ seven, you get out of bed at exactly seven o’clock. Not before or after seven, but exactly at seven. When you wake up `by’ seven o’clock, you get out of bed at seven or a few minutes before seven.
Examples: I told my mother that I would be home by 6:00 o’clock.* *Ganesh said he’d be back home by 7: 00 o’clock. He returned at 10: 00 o’clock