As we are primarily active during daylight hours, our bodies have evolved a schedule for different metabolic processes to make sure we use energy in the most efficient way. Muscle performance and reaction times are optimised during the day, while tissue repair and memory formation are handled during our downtime. The circadian rhythms are the 24-hour cycles of each part of our body. They are controlled by the fluctuating levels of different signal molecules in our cells – known collectively as biological clocks. All the clocks are synchronised by a master clock in our brain called the suprachiasmatic nucleus, which is a group of around 20,000 brain cells in the hypothalamus.