HOW CELLS TELL TIME

Many physiological phenomena are rhythmic and coincide with a particul ar time of day. These 'circadian rhythms' are Mot dependent on externa l timing cues but are driven by infernal circadian clocks that are ubi quitous features of living organisms. Although many of these rhythms m anifest themselves as complex behavioural patterns, we now know that a circadian clock does not require a complex organism or an elaborate n ervous system; if can be built from molecules within an individual cel l. This review focuses on new advances in identifying and understandin g the basic properties of cellular circadian clocks.