Fig. 1.

Input and output pathways to/from the suprachiasmatic nuclei (SCN). The photic input pathways that relay information about the intensity and spectral composition of ambient light are the retinohypothalamic tract (RHT) and the geniculohypothalamic tract (GHT), which connects retina and SCN via the intergeniculate leaflet (IGL) in the thalamus. Additionally, the SCN also receive non-photic information from the raphe nuclei (RN) via the raphe-hypothalamic tract (raphe-HT) and from the pineal gland. The main output is from the SCN to the serotonergic raphe nuclei (RN, receive information about the phase of the circadian clock and regulate vigilance state of the body) and the pineal gland, where melatonin is produced. Input and output pathways form reciprocal loops