A circadian rhythm is superimposed on the homeostatic mechanism. Circadian rhythms are controlled by the suprachiasmatic nucleus, which emits alerting signals; this signal is believed to be attenuated by melatonin, which is produced in response to darkness. It is believed that binding of melatonin to MT1 and MT2 receptors in the suprachiasmatic nucleus inhibits firing of specific neurons, and this is thought to attenuate the alerting signal, allowing the homeostatic mechanism to express itself and promote sleep.