Pinealocytes, neuron-like cells in the Pineal Gland, produce melatonin which is derived from serotonin. They induce rhythmic changes in the body in response to light. Melatonin levels rise at night and make us sleepy. Melatonin plays a role in our light-dark cycles and its secretion (or lack thereof) is one basis for our feelings associated with jet lag.
The responses generated by the Pineal Gland and its secretions make it the biological clock of the body.