Suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), is a small nucleus in the hypothalamus. It is in middle of the brain. It sits on top of the optic chiasma. 
It is our "body clock", and controls our daily rhythms by hormones and nerve impulses. With its various cell types, it puts out peptides such as vasopressin and vasoactive intestinal peptide) and neurotransmitters.
The SCN controls when we get sleepy, when we wake up, when we are active and when we get hungry. The SCN gets information about light and dark from the retina in the eye so it can adjust itself to the true 24 hours. To do this it uses only about 20,000 neurons.
The SCN also exchanges information with many other parts of the brain. It uses many different types of cells and many different types of peptides (including vasopressin and vasoactive intestinal peptide) and neurotransmitters.