Like the behaviour and physiology of the organism, biochemical parameters are subjected to diurnal variations through the daily 24-h light and dark cycle. Since blood sampling is carried out at all hours of the day and night, repeated blood testing from a patient throughout the day can be biased by diurnal variations, especially in parameters with high-amplitude 24-h rhythms. Concentration changes in biochemical parameters have thus to be interpreted in view of the 24-h rhythm, as only few recommendations regarding the optimal time of venipunctures and time-specific reference intervals exist.