This week Science Weekly is dedicated to the vital but often neglected topic of sleep and its importance to health, wellbeing and mental sharpness. Prof Russell Foster discusses recent developments in our understanding of sleep cycles and how our body clock or "circadian rhythm" is programmed at a cellular level.

We discuss how our eyes entrain the circadian rhythm and why this ancient biological system is common to all living organisms.

Prof Foster has written about the importance of sleep for guardian.co.uk.

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