Every day we experience profound changes in our mental and physical condition as body and brain alternate between states of high activity during the waking day and recuperation, rest, and repair during night time sleep. These cycles are not a passive response to the world around us: they are pre-adapted, driven by an internal clock. We know this because when human volunteers are held in experimental isolation and deprived of any temporal or social cues, they still show daily cycles of sleep and wakefulness, in core body temperatures, and urinary output (fig 1). 1^–2 As with all biological processes, the clock driving these cycles is slightly imperfect, therefore the measurable rhythms free run with periods of slightly less than or greater than one solar day, hence circadian (approximately a day). Notwithstanding this inaccuracy, the circadian clock is extremely robust. It is capable of continuing for several months and with a reproducibility to within a few minutes per cycle.