Many of the body’s rhythms reflect the circadian cycle, but how do they interact? ‘Light and dark’ describes a network of systems involving the senses, parts of the brain, the activation and inactivation of genes, biochemical reactions, and cellular responses, all coordinated by complex hormonal cycles. Melatonin, a hormone from the pineal gland, tracks darkness: concentrations are high at night and low during the day. Melatonin cycles affect reproduction and many other physiological changes in animals and it can also slow the division of cancer cells. The circadian connection to the important hypothalamic–pituitary–adrenal axis and the hormone cortisol is also explained.