The mammalian circadian clock is believed to be performed by a hypothalamic tissue called suprachiasmatic nucleus (SCN) which is called as such because it resides right next to the optic chiasm. It is composed of tens of thousands of interconnected neurons, a fraction of which each exhibit circadian oscillation in electrical activity and expression of some genes. Even though these neurons do not have similar period, their interaction causes them to synchronize with each other thereby producing a stable circadian period of the whole SCN. The mechanism behind the individual oscillations and how they interact with each other is still not completely known. Watch the following 2-min lecture on the SCN from the Howard Hughes Medical Institute.