How do people wake up just before their alarm goes off?
The biological answer is presumably cryptochromes.  The foregoing applies to humans who have CRY2 blue light receptors in the inner retina (the neural layer, not the photoreceptors).  I've seen claims these respond particularly well to the light before dawn, though I didn't track those down just now.
That said, a proper skeptic would dismiss this as confirmation bias, at least until proven otherwise.