Book excerpt: Why am I always tired? Why can't I sleep at night? Why do I suffer from jetlag? We all have a body clock, a biological structure that controls how we feel, our mental and physical performance, and whether we are active or asleep. This is nature's response to our rhythmic environment, dominated by day and night. Though the body clock normally adjusts us to daytime activity andnight time sleep, it can go wrong or be tricked by lifestyle changes. This can result in jetlag, some forms of insomnia and even depression. Keeping in time with your body clock provides clear and accessible advice on how to live with, and not against, your body clock. In a clear and accessible style, it explains how the body clock works, how and why it can work against you, and the measures you can take to optimise your feeling ofhealth and wellbeing. It also explains the role of the body clock in illness, and how an understanding of this can increase your feeling of health. An essential book for anyone who wants to better understand their body and optimise their feeling of health and wellbeing.