Our lives are ruled by time and we use time to tell us what to do. But the digital alarm clock that wakes us in the morning or the wrist-watch that tells us we are late for supper are unnatural clocks. Our biology answers to a profoundly more ancient beat that probably started to tick early in our evolution. Embedded within the genes of us, and almost all life on earth, are the instructions for a biological clock that marks the passage of 24 hours. Biological clocks or circadian clocks help time our sleep patterns, alertness, mood, physical strength, blood pressure and much more. Under normal conditions we experience a 24 hour pattern of light and dark, and our circadian clock uses this signal to align (entrain) biological time to the day and night. The clock is then used to anticipate the differing demands of the 24 hour day. Body temperature drops, blood pressure decreases, cognitive performance declines and tiredness increases in anticipation of going to bed. Whilst before dawn this process is reversed. This presentation will consider the mechanisms of how this “day within” is generated and regulated by light and will review how our health can be affected when this system becomes disrupted (e.g. shift work) or stops functioning normally (e.g. schizophrenia).