Light and temperature provides the determining factor by which the body responds to the 24-hour activity period
In complex animals like humans, the hypothalamus forms part of the autonomous nervous system which in part, controls the functions of organs that are not voluntarily controlled, hence involuntary. Involuntary, meaning, they are not governed by conscious will to act. These include breathing, heart beat and intestinal digestion.
One interesting point to make is that the location of SCN is just above the optic chiasma (cross). The optic chiasma can be described as a location where optic nerve fibers meet and cross (chiasma-cross).
It is fitting to say that, the perception of light through the opening in the eyes, triggers the wake/sleep patterns in humans since this is one way the organism receives such physical message.
As the retina (a portion of the eye where initial image of an object is stored) traps light rays, optic fibers relays the image to the brain via characteristic electrical pulses.
This unique sensory impulse is carried along the optic fibers to the occipital lobe (rear part) of the brain and is perceived as images. The proximity of the optic chiasma to the SCN perhaps provides the opportunity for the hypothalamus to “sense” the referred electrical impulses.