The human pineal gland is an extremely active neuroendocrine transducer. Environmental light acts through the retina and entrains the pineal gland's circadian rhythms by way of the hypothalamus and sympathetic nervous system. Light depresses pinealocyte activity. It is possible to do without the pineal, but this tiny gland is considered to be the "regulator of regulators" and important in general homeostasis. A direct retino-hypothalamic pathway is probably involved; and a system of synchronizing potentially independent oscillators is postulated.