Several years ago, flying back to ATL, very early in the morning. ATC: "DAL 1234, decend to FL240." No response. ATC repeats, no response. ATC repeats again, no response. Finally, over the radio, "Beep-beep-beep-beep-beep-beep (an alarm clock), DAL 1234 ATL center, this is your wake up call, good morning."