This is the process by which circadian cycles are set by exposure to light, most typically sunlight.

In mammals, a tiny fraction of the light information gathered at the retina is shunted away from the primary visual information pathway and directed towards the suprachiasmatic nucleus.* This nucleus is the circadian pacemaker by virtue of its connectivity with the pineal gland and many other subcortical brain structures -- and thus light is the primary zeitgeber in mammals.

To be fair: in humans and other higher mammals, the circadian cycles can be entrained in many, many ways: food administration, social cues, ambient heat, ambient noise, etc. In fact in the absence of any cues, scheduled melatonin administration can be used to entrain circadian rhythms. However, light cycles will override any of these secondary cues, and can be used to extend or shrink the period of the natural human circadian rhythm (24-25 hours) by several hours with little adverse effect.

And: In birds, photoentrainment cues aren't related through the eye but are actually picked up by a small group of photoreceptors located inside the brain -- they are sensitive to the tiny amount of light that penetrates through the skull. Oh yeah, and plants do this too: see photoperiodism.

*More accurately: A small number of axons that leave the retina do not follow the optic nerve, but rather form a separate projection towards the suprachiasmatic nucleus called the retino-hypothalamic tract.

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