The suprachiasmatic nucleus (SCN) is a group of nuerons in the hypothalamus of the brain that is known as “master clock” of circadian rhythm. It is located above the optic chiasm and has direct connection to the retina of the eye to detect light. Or more properly stated …
“The SCN receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract.”
Reference: Wikipedia
So the light signals that it receives, helps synchronize the master clock.
But in addition, the suprachiasmatic nucleus also has a time-ticking mechanism of its own.
From Dr. Dement’s book The Promise of Sleep, it writes …
“The timekeeping mechanisms of the biological clock seems to be based on the movement and chemical interactions of molecules of in the SCN.”
In the biological clock of fruit flies, they found…
“The clock proteins build up inside the cell until they reach a critical mass. Once enough clock protein builds up in the cell, it travels to the cell nucleus and turns off its own production. As the existing proteins disintegrate naturally, their ability to suppress their own production fails, and their production begins again. The sets up a natural oscillation of protein production and disintegration.”
Dr. Dement writes …
“It may seem a wonder that something nonmechnaical can measure time so accurately, down to a few minutes over the course of 24 hours.”
In the book, The Brain’s Way of Healing, it writes …
“Alongside the retinal cells we use for seeing — our rods and cones — other light-sensitive cells were found that send electrical signals on a separate neuronal pathway, also in the optic nerve, to a clump of cells in the brain called the suprachiasmatic nucleus (SCN), which regulates our biological clock.”