We do not often give thought to the factors controlling our circadian rhythm. And yet the circadian system modulates many physiologic systems, including brain arousal, cardiovascular function, sympathetic tone, appetite, metabolism and immune system function. Similar to the sinoatrial node pacing the heart, the suprachiasmatic nucleus located in the hypothalamus serves as the central pacemaker for the circadian rhythm, directing sleep, motor activity, temperature and autonomic tone. Rhythmic release of melatonin from the pineal gland helps drive this central clock in addition to other circadian biomarkers, including cortisol and core body temperature. In healthy individuals, plasma melatonin concentrations typically measure 10-fold higher at night than during the daytime. How does critical illness affect circadian rhythm, specifically melatonin secretion?