Light cones aren’t exactly literal cones of time, they are an abstraction to help us understand the mathematics of time and space. (Assuming you are talking about Penrose diagrams.)

You might be waking up in a more ideal spot in your REM cycle. For me if I get 6 ½ hours I feel great but I feel like shit if I get a full 8. But if I sleep 9 I feel pretty good again. If you wake up and get interrupted during REM you’ll feel a lot more groggy and like shit for most of the day in my experience.