AbstractThe seasonal change in the duration of the night time release of melatonin is responsible for activating the hypothalamic gonadtrophin‐releasing hormone (GnRH) pulse generator in seasonal breeding animals. Paul Mullen and I considered that it might also be responsible for the activation of the child's GnRH pulse generator at the onset of human puberty. The real conundrum, though, is why this should happen when it does. Homeostasis, which explains how the body is able to maintain itself, cannot explain why things change. Our reasoning was that in any homeostatic state, there might exist two systems in disequilibrium, which eventually collide to create the ‘catastrophe’ of change. Our research showed that a constant daily output of melatonin throughout life was in disequilibrium with increasing body mass throughout childhood. These two systems permit the progressive decrease in the circulating concentration of melatonin in the growing child until it drops below a critical threshold, which then creates the ‘catastrophe’ of puberty. Catastrophe is destined to overthrow the established order, whereas homeostasis protects it. Catastrophe is always a transient moment that is rapidly supplanted by the re‐imposition of homeostasis. Copyright © 2010 John Wiley & Sons, Ltd.