I have been using one of those sunrise alarm clocks for around 8 years now. If I go to bed at a reasonable time, the light will wake me up naturally before the alarm actually goes off. However, if I stay up too late the effect of the light fades. I'll sleep right through the light and need the alarm. On one occasion when I must have been really sleep deprived, I slept through the light, an hour of the alarm going off, and another 2-3 hours after that.