Daily rhythms of activity and rest in animals are usually well coordinated with environmental cycles of light and dark. However, research in the past proved that environmental cues are insufficient to determine this behavior. Work in rodents has identified a specific brain region, the suprachiasmatic nucleus (SCN), that possesses circadian (approximately 24-h) clocks. Loss of these tissues produces arrhythmic behavior even in the presence of a day-night cycle. Cycles of sleep and wakefulness are restored by transplanting the suprachiasmatic nucleus from a healthy brain; environmental cycles are not needed for the production of the rhythmic behavior. When individual cells of the suprachiasmatic nucleus are propagated outside the animal, they produce approximately 24-h cycles of the electrical and metabolic activity, indicating a molecular mechanism that can generate a cellular circadian rhythm.