Melatonin, a neurohormone, plays important roles in adjusting the 'biological clock', and is an important mediator in many normal physiological functions [1]. Melatonin disturbances have been linked to neuropsychological dysfunction and postoperative delirium [2]. However, there have been no reports on postoperative melatonin levels to date. The aim of the present study was to observe perioperative melatonin secretion in patients undergoing coronary artery bypass graft surgery.

After local ethics committee approval and informed consent, 11 male patients aged between 60 and 78 years, who were scheduled for elective coronary artery bypass graft surgery under hypothermic cardiopulmonary bypass, were enrolled in the study. All patients received the same anaesthesia regimen, which is used routinely at Papworth Hospital. Blood samples for measurements of melatonin were taken during the day of surgery at specific time points and every 3 h on postoperative days 2 and 3. Plasma concentrations of melatonin were measured using a radioimmunoassay method.

During surgery melatonin levels were below the minimum sensitivity level in most patients, but low levels, without circadian variation, were measured during the first postoperative night in some patients (n = 7; Fig. 1). Circadian secretion was observed on the third postoperative night, irrespective of whether the patient was on the ward (n = 6) or in the intensive care unit (ICU; n = 3; Fig. 1). However, the peak level of melatonin was observed 3 h later in patients who remained in the ICU (Fig. 1).