The regulation of biological clock in higher vertebrates is done by __________.

The primary circadian clock in mammals is located in the suprachiasmatic nucleus (or nuclei) (SCN), a pair of distinct groups of cells located in the hypothalamus. Destruction of the SCN results in the complete absence of a regular sleep–wake rhythm. The SCN receives information about illumination through the eyes. The retina of the eye contains photoreceptors (rods and cones), which are used for conventional vision. But the retina also contains specialised ganglion cells which are directly photosensitive, and project directly to the SCN where they help in the entrainment of this master circadian clock.