The nucleus, which is located in the hypothalamus just on top of the optic nerve cross (chiasma opticum), consists of about 20,000 neurons with very different functional properties.
The nucleus conducts the body's circadian rhythm with a periodicity of just over 24 hours. The day/wakefulness and night/sleep sections are controlled partly by an intricate innate genetic machinery and partly directly by the influx of light to the eye.
Some of the ganglion cells of the retina - i.e. some of the nerve cells whose axons form the optic nerve - are in themselves directly sensitive to light (probably due to their content of the photopigment melanopsin). The axons of the light-sensitive ganglion cells form the retino-hypothalamic conduction pathway (tractus retino-hypothalamicus). The conduction pathway forms part of the optic nerve and ends mainly in the suprachiasmatic nucleus.
The suprachiasmatic nucleus, in turn, sends its rhythmically varied messages during the day, partly as nerve impulses to a large number of areas of the brain itself, including the noradrenaline nucleus and serotonin nuclei in formatio reticularis, and partly as various homonellic control signals, e.g. to adjacent nuclei in the hypothalamus and to various organs outside the nervous system.
The suprachiasmatic nucleus controls, not entirely unexpectedly, the diurnal variation in the pineal gland's production of the hormone melatonin. Melatonin production is highest during the "dark" sleep/rest period of the day and melatonin is considered important for a normal night's sleep.
The production of melatonin is controlled by a rather peculiar coupling system:
a/ From SCN, nerve signals emanate through several switches that travel through the brain stem and down the spinal cord to the upper part of the sympathetic spinal cord nucleus.
b/ Here the signal is switched on preganglionic sympathetic neurons.
c/ These send their axons out into the sympathetic boundary cord and up to the upper cervical ganglia where the signal is again reconnected, now to postganglionic sympathetic neurons.
d/ From the upper cervical ganglia, the postganglionic threads, with the help of the internal carotid artery and its branches, finally find their way to and into the pineal gland.
e/ The secretion of norepinephrine from the postganglionic nerve endings stimulates the cells of the pineal gland to produce melatonin.
The circadian comes from the Latin circa diem = about a day (a day).