Biological rhythms are an essential component of homoeostasis: 'everything is rhythmic unless proved otherwise'. Circadian (ca 24 h) rhythms are driven by an internal clock situated in the hypothalamus. Without time cues, they deviate from 24 h and assume a periodicity of, on average, 24.3 h. Periodicity is inherited, and progress has recently been made in identifying 'clock' genes. Circadian rhythms are synchronised to the 24-h day mostly by light-dark cycles and social time cues. Suitably timed bright light and the pineal hormone melatonin will change the timing of circadian rhythms and can be used to treat biological rhythm disorders, whether occupational or pathological. Rhythm disorder is usually manifested as sleep problems, but some major pathology (eg cardiovascular disease) is associated with frequent rhythm disruption as found in shift workers. Menstrual cycle variations are clearly of importance, and even human seasonality must be taken into consideration. Many drugs show rhythmic variations in pharmacokinetics and pharmacodynamics. Exploitation of this phenomenon, together with underlying target tissue sensitivity, is underway.