An internal 24-hour biological clock (circadian clock) controls, modulates and fine-tunes our sleep patterns, alertness, mood, physical strength, blood pressure, and every other aspect of our physiology and behaviour. This ‘day within’ even changes our responses to drug treatments which can show large time-of-day effects. Under normal conditions we experience a 24-hour pattern of light and dark, and our clock uses this signal to align biological time to the day and night. The clock is then capable of anticipating the differing demands of the 24-hour day and adjusting our biology in advance of the changing conditions. Body temperature drops, blood pressure decreases, tiredness increases in anticipation of going to bed. Whilst before dawn, metabolism is geared-up in anticipation of increased activity. The past decade has witnessed remarkable progress in understanding the mechanisms that generate circadian rhythms and sleep. We now know where in the brain the ‘master’ clock is located and how individual cells can generate molecular rhythms. We also know that the eye contains specialised light detectors, different from the cells that regulate vision, that detect the dawn/dusk signal. In parallel with our increased understanding of mechanisms, there is a growing appreciation of the severe consequences of ignoring the impact of these rhythms on our physiology, health and quality of life. The presentation will consider how circadian rhythms are generated, regulated by light and why we can’t ignore our internal time in both medical treatments and in the way we organise our 24/7 society.