The philosopher Descartes suggested that the pineal gland was the seat of the soul (1). Modern research has revealed that it plays a key role in setting the daily sleep-wake cycle (circadian rhythm). Melatonin, a hormone released from the pineal gland, is crucial for the maintenance of a healthy circadian rhythm, and its highest level in the blood is normally observed during darkness (2). However, a reduction in nocturnal melatonin levels has been observed in patients and animal models with cardiac disease (3) and could be responsible for associated sleep disorders, including difficulty in initiating and maintaining sleep. On page 285 of this issue, Ziegler et al. (4) describe a pathway by which cardiac disease leads to immune-mediated sympathetic denervation of the pineal gland and a subsequent decrease in circulating melatonin, causing sleep disruption. This provides a link between cardiac disease and sleep disorders and identifies an additional connection between the immune system and sympathetic function.