The suprachiasmatic nucleus (SCN) portion of the brain functions as the master circadian pacemaker and is a major influence on sleep patterns and the sleepiness/awakeness continuum.
The body responds to light and dark cycles of the environment, and the eyes are involved in this process. Curiously, the regular photoreceptor cells in the retina (the rods and cones familiar from biology class) do not seem to be the only thing working here. Experiments with mice have shown that even animals with no rods or cones pick up on the circadian cues of light and dark in the environment. Scientists have shown that there are some receptors in the retina that contain the pigment melanopsin and that these receptors activate the SCN. This process is called "nonvisual photoreception."
Opsins are proteins in the retina. Rhodopsin, for instance, changes shape in response to blue light. Other opsins respond to different wavelengths. Melanopsin changes shape in response to blue light to a configuration that reduces resistance to electrical current.
When the SCN is called the master pacemaker, it is because it influences externally observable behavior (e.g. eating, falling asleep and waking up) as well as physiological measurements (e.g. corticosterone levels in the blood).
A deeper understanding of these photoreceptors may provide new ways to overcome jet-lag and treatments for disorders such as SAD (Seasonally Affective Disorder) that can be caused by a lack of light, particularly during the winter months.
Scientific studies have revealed relatively low levels of amino acid sequence conservation across species for this protein compared with that reported for photopigments. This suggests that the entrainment of the circadian phase to sunlight is very ancient in evolution.
Melanopsin absorbs light and triggers a chemical cascade that signals the brain about external light levels. Intrinsically photosensitive retinal ganglion cells synchronize the body’s daily rhythms to the rising and setting of the sun.
ipRGCs were only discovered in the early 20th Century; a decade later another shocking discovery was that they might play a role in memory. Scientists found evidence that melanopsin established "photic memory" that may play a part in cognition.
Related: Molecular circadian rythyms