Almost 30 years ago, researchers in the US made the astonishing discovery that destroying a very small structure near the ventral surface of the brain (Figures 1A and 1B), the suprachiasmatic nuclei (SCN) of the hypothalamus, abolished daily (near 24 hour or circadian) behavioural rhythms in rodents1. In intact animals, such oscillations in behaviour and endocrine cycles are sustained when animals are kept under constant conditions (i.e. constant dark, constant temperature, etc.), demonstrating that they are endogenously generated. Remarkably, grafting foetal SCN tissue into the brains of SCN-lesioned arrhythmic adult animals rescues behavioural rhythms2, confirming that the SCN are indeed the sites of the brain's master circadian clock.

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