The pineal gland contains a circadian pacemaker in some vertebrates, such as perching birds (passerines).

In humans this function is taken over by the SCN in the anterior hypothalamus, which controls the cycle of pineal melatonin secretion.

The human pineal gland contains glial cells and pinealocytes which have similarities to the photoreceptors of amphibia, but in humans they are only indirectly, not directly, light sensitive. The pinealocytes secrete several hormones and related chemicals such as GABA and 5HT, but their most important hormone is melatonin.

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