The circadian clock is a biological system that allows organisms to adapt to temporal constraints of the environment. It governs all body functions. The circadian clock has an endogenous period and entrains to the external day by the means of timing cues, also called "zeitgebers". A discrepancy between internal and external day or internal day and certain body functions results in circadian desynchrony, most prominently represented in jet lag. Circadian desynchrony may be a factor in many diseases. This review presents literature in which the impact of circadian desynchrony on disease development - with a special focus on critical illness - has been addressed. The treatment options that have so far been investigated are also presented.