Body weight and fatness are influenced by day-length. We have studied energy expenditure via the production of heat by skeletal muscle (through thermogenesis). This project, will determine whether muscle thermogenesis is affected by day-length, establishing a new dimension in our understanding of weight regulation. The results will be important for livestock production by ascertaining changes across the year and will also have implications for human health, because disruption of the normal day/night cycle increases obesity susceptibility. We predict that reduced muscle thermogenesis will be causally linked to increased body weight and fatness. The project will delineate mechanisms underlying seasonal pattern changes in energy expenditure.