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According to Douglas Adams, “Time is an illusion. Lunchtime doubly so.” But humans seem to have an obsession with time. Since time immemorial, we have tried to find ever more accurate ways of measuring its passage. From the first shadow clocks, around 5,000 years ago, to the atomic clocks of the 20th century, our timekeeping devices have grown increasingly sophisticated.

But we also have a surprisingly advanced internal timekeeping apparatus. As Buhusi and Meck describe on page 755, the human brain can measure time across scales from milliseconds to days. They focus on one type of timekeeping, known as interval timing. This process, which covers durations from seconds to hours, is crucial for many types of behaviour and is found in many animals, including birds, fish and mammals.

The cerebellum is thought to be involved in millisecond timing, and the suprachiasmatic nucleus drives the 24-hour circadian rhythm in mammals. By contrast, interval timing seems to involve a more distributed system. Traditionally, interval timing has been ascribed to a 'pacemaker–accumulator' model, but the authors propose that this distributed network tells the time by detecting the coincidental activation of neurons.

Compared with our understanding of the circadian rhythm, we know little about how interval timing works at a neuronal or molecular level. Genetic techniques unravelled the mechanisms of circadian timing, and might also reveal the molecular basis of other types of timing. Modern techniques, such as computational models, should also shed light on our internal clocks.

Finally — on the subject of time — we are delighted to be celebrating the fifth anniversary of the launch of Nature Reviews Neuroscience. Many thanks to all of our authors and referees, who, together with our readers, have made the last 5 years a true success.

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In this issue. Nat Rev Neurosci 6, 745 (2005).

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