The medial zone is divided into three regions: supraoptic, tuberal, and mammillary. There are four nuclei in the supraoptic region, which play a role in thermoregulation, osmoregulation, and the sleep–wake cycle. The supraoptic and paraventricular nuclei in this region synthesize antidiuretic hormone or vasopressin, which stimulates water uptake and oxytocin for stimulation of uterine contractions and lactation in the mammary glands. Axons from these nuclei are conveyed by the hypothalamohypophyseal tract to the posterior pituitary for release into the circulatory system (eFig. 9.89). A third nuclear group in this region is the suprachiasmatic nucleus. This nucleus receives input directly from the retina to influence circadian rhythms, which contribute to the light–dark cycle. The anterior nucleus is the final group and functions predominantly in regulating body temperature.