Chronobiology investigates biological rhythms - predominantly those that correspond to environmental cycles (tidal, daily, lunar or annual). The most intensively investigated biological timing system is the circadian clock. Behaviour, physiology, bio-chemistry, and gene expression are temporally structured over the course of a day, generating daily oscillations at all levels of life.

Cycles in attention and activity, in core body temperature and heart rate, in enzyme activity and hormone levels are not simply driven by external changes (e.g., light/dark or warm/cold). Rather, they are controlled by an endogenous, circadian clock which are found in all phyla - from cyanobacteria to humans. Daily rhythms continue in the absence of environmental signals with their own periods that can deviate slightly from 24 h - hence the term circadian (about one day). Although circadian rhythms are endogenous and self-sustained, the ‘internal day' is regulated by external cues: circadian rhythms are synchronised (entrained) by and to the periodic environment. This ‘entrainment’ uses environmental signals (predominantly light) as 'zeitgebers'.

Over the past decades, circadian research has unravelled many details about how the circa-24-hour rhythmicity is generated at the cellular level. Clock genes and their products form a complex network of feedback loops. Approximately 10% of the mammalian genome shows a circadian rhythm in its expression.

The programme of the Gordon Conference on Chronobiology in 2007 covers the most recent insights into the mechanisms of circadian rhythmicity, ranging from genetics, molecular biology and neurophysiology to higher functions such as sleep and performance in shift work. The contributions reflect the need for our field to show how these temporal programmes function in the real world under entrainment as well as the necessity to explore clock mechanisms beyond the known transcriptional-translational feedback loops.

Because of the enormous and predictable daily changes in the metabolism of each cell, insights into circadian clocks are essential for medicine as well as for coping with the consequences of life in an industrial society that has us living indoors (i.e., providing weak zeitgebers), performing shift work and incurring jetlag.