a | Light activates the glutamate (GLU)-containing retinohypothalamic tract (RHT) that runs from the eye to the suprachiasmatic nucleus (SCN). Through a polysynaptic projection, the SCN functionally inhibits the activity of the superior cervical ganglia (SCG), which supply the pineal gland with an excitatory, noradrenaline (NA)-containing input. This circuit allows light to suppress the production and release of melatonin from the pineal gland and, correspondingly, melatonin secretion is increased in the dark period. Melatonin reciprocally activates neurons in the SCN by actions at melatonin 1 (MT1) and MT2 receptors. Serotonergic input from the raphe nucleus modulates the SCN through actions at serotonin (also known as 5-hydroxytryptamine; 5-HT) receptor 5-HT2C and other classes of 5-HT receptor. Daily behaviours likewise influence output from the SCN, the neuronal master clock for coordinating circadian rhythms. b | Melatonergic receptors recognized autoradiographically in the SCN using [125I]iodomelatonin. c | Locomotor activity rhythms of rats drift backwards when the onset of the dark period is delayed by several hours. Daily administration of agomelatine (3.0 mg per kg, intraperitoneally) resynchronizes rhythms to their usual circadian pattern (dark period commencing at 18:00 hours)45.