When suprachiasmatic nucleus (SCN) neurons receiving the retinohypothalamic tract (RHT) are activated, they act to increase adrenaline and glucagon secretions and decrease insulin secretion through the neural networks. The dorsomedial subdivision contains the neuronal cells involved in generation of the circadian time signal, while the ventrolateral subdivision contains the cells involved in regulation of energy metabolism. The combination of changes in hormonal secretions and activities of the autonomic nervous system result in alterations in activities of rate-limiting enzymes of carbohydrate metabolism in various tissues. Neural activity of the SCN is higher during the light period than during the dark period. The regulatory mechanism of the SCN can also operate without direct relationship to the circadian oscillator, under the influence of information transmitted to the SCN, secondary to changes in external and internal environmental stimuli. The reduction of protein intake associated with aging might be induced by lowering the neural activity of the SCN neurons.