Suprachiasmatic nucleus

The circadian clock that regulates sleep–waking cycles resides in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The clock function of the SCN is due to pacemaker neurons which fire with a frequency that vary in a circadian fashion even when isolated from the rest of the nervous system. The firing frequency of SCN neurons varies sinusoidally with a period of 24 hours peaking in the duration of the day and dropping to its lowest rate during the night. SCN projects hugely to other hypothalamic structures to regulate autonomic, endocrine functions and sleep–wake cycles, but also sends output to the thalamus and basal forebrain for example septal nucleus that probably accounts for circadian variation in cognitive and memory functions. Most SCN neurons are GABAergic and co-release peptides and are assumed to be inhibitory on their goals.

Light signals encoding total luminance are relayed to the SCN through the retinohypothalamic tract (RHT). This pathway consists of the axons of a population of small retinal ganglion cells driven through cone photoreceptors over a broad area that synapse stately with neurons in the core of the SCN. RHT uses glutamate as a transmitter.