There are three types of photoreceptor cells in the human retina: rods, which see black and white and have peak sensitivity to (green) light at 506 nm; cones, which see color and have peak sensitivity to (green-yellow) light at 555 nm; and photosensitive retinal ganglion cells (pRGC), which have peak sensitivity to short-wavelength blue light. The pRGC contain the blue-light sensitive photopigment melanopsin in their cell bodies and elongated dendrites. Spectral absorption by melanopsin and sensitivity of human nocturnal melatonin suppression both peak in the blue portion of the spectrum at 480 and 460 nm, respectively.Ganglion photoreceptors send unconscious, non-visual photic information through the retinohypothalamic tract to the SCN permitting alignment of internal biological with external environmental time. They differ in many ways from the rods and cones that subserve conscious image-based vision. Ganglion photoreceptors require much more light to respond than cones and have thresholds well above those for photopic vision. They lack spatial resolution and can adapt to ambient lighting over days and months. These properties are well suited to non-directional detection of gross environmental illumination essential for integrated circadian, neuroendocrine and neurobehavioral effects. Absent or deficient pRGC photoreception cannot be perceived subjectively, but ensuing circadian disturbances can have significant physiological and psychological consequences.
The SCN initiate events timed to allow preparation for impending metabolic, biochemical and physical activities. Prior to awakening, they activate a morning cortisol surge and trigger changes vital to transitioning from sleep to wakefulness. Morning exposure to sunlight increases core body temperature, alerting, cognition and brain serotonin levels which enhance mood and vitality. As the day progresses, peak cognition occurs commensurate with maximal core body temperature. By evening, SCN actively inhibit cortisol secretion for recovery from the morning surge and initiate pineal secretion of the hormone melatonin which reduces alertness and decreases core body temperature. As sleep ensues, its slow wave stages and SCN suppression reduce cortisol to a healthy daily nadir as SCN orchestrate a nightly surge of melatonin and other sleep-related hormones.
Excerpted and adapted from: Circadian photoreception: ageing and the eye's important role in systemic health