How the mammal internal body clock develops

The suprachiasmatic nucleus (SCN) shows circadian rhythms in clock gene expression from the embryonic stage throughout postnatal life.

• The mammalian circadian system includes a central clock in the hypothalamic SCN and peripheral clocks in various tissues.
• Most SCN neurons possess individual circadian clocks with specific periodicity, critical for synchronizing peripheral clocks.
• Photic entrainment of the SCN begins postnatally, gradually establishing its role as the central clock controlling rhythms.
• Changes in the SCN networks occur in postnatal weeks 2-3 as indicated by the loss of coherent circadian rhythms in cryptochrome-deficient SCN.
• Different clusters of cellular circadian rhythms in the SCN are integrated by specific signaling molecules during postnatal development.

Simplified