In the eye there are photo receptors that respond to light, I think that much is obvious, but more specifically there are specific types of photoreceptive cells called retinal ganglion cells (RGCs) which contain melanopsin, a pigment that is energized by a certain wavelength of light. That energizing causes the the RGC to send a neural pulse, called an action potential to a region of the brain called the suprachiasmatic nucleus (SCN), located near the hypothalamus. Now, interestingly while RGCs do play a role in sight, this specific pathway to the SCN is different than the pathway to the optical cortex(which is at the back of the head, not near the hypothalamus), and is why some blind people can still respond to light and maintain a circadian rhythm without being able to actually see or process what they are seeing.

Now, this is complex and it’s been a minute since I studied the specific pathway, so I’ll explain it incredibly simply. Once the action potential reaches the SCN, it innervates cells there, which is where the proteins I mentioned are created. Those proteins act as transcription factors, which cause the transcription of many genes essential for sleep, such as melatonin. And I think that’s where I’ll leave it, as I’ll get it too convoluted, and I’ve probably already said some things wrong. Just know that’s generally how the system functions, and that it’s the SCN that acts as the hub which releases hormonal signals to synchronize the entire body’s cells individual clocks.

If you’re interested in the subject, there are much more in depth articles and videos on the circadian clock I’d recommend looking into.