Circadian clockwork: two loops are better than one.

The spectacularly successful race over the past three years to place our understanding of the circadian clockwork of mammals into a molecular framework is beginning to yield the cardinal example of the molecular-genetic control of behaviour. This perspective describes recent evidence for the conservation of a double-loop, autoregulatory feedback mechanism across the best understood eukaryotic circadian systems, and discusses how these findings may illuminate some long-standing puzzles concerning our subliminal sense of circadian time.