The suprachiasmatic nucleus (SCN) portion of the brain functions as the master circadian pacemaker and is a huge influence on sleep patterns and the sleepiness/awakeness continuum cycle.

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 pigment is an opsin-like protein and they are calling its action “nonvisual photoreception.”

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 other opsin photopigments.

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.

Related: Molecular circadian rythyms

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Hypnogogia

"O Sleep, rest of all things, mildest of the gods, balm of the soul..."

(Iris to Hypnos. Ovid, Metamorphoses)