This has also worked for me. But I'm not sure if it would have worked without the alarm clock I have. It has the feature that it starts to shine 30 minutes before the alarm, getting brighter before starting to ring.

If the alarm clock is shining, I know it's time to get up. If it's not shining, I know there's enough time to sleep more. I don't need to worry I'll fall asleep 2 minutes before the alarm ringing.