Daily fluctuations in the efficacy and toxicity of drugs are thought to reflect effects of the circadian clock on disease severity, target organ sensitivity, and the absorption, distribution, metabolism, and excretion (ADME) of drugs. A better understanding of circadian clocks would permit the timing of drug delivery to be optimized in relation to these factors and may lead to new therapeutic strategies in which the timing of the circadian clock in a selected target organ or tumor is reset pharmacologically in order to sensitize the tissue to a subsequent medication. Some existing drugs may have unforeseen effects on physiology by influencing the timing of circadian clocks. Informative biomarkers that monitor the phasing of circadian rhythms in peripheral organs in man are urgently required...