Melatonin, an endocrine product of the pineal gland, is formed predominantly during the nighttime. Light has an inhibitory effect on pineal melatonin secretion. Pineal melatonin release is synchronised by this daily light-dark cycle via a multisynaptic pathway between the eyes and the pineal gland. Light stimulates the retina to modulate the activity of the suprachiasmatic nucleus, the master biological clock.1 The suprachiasmatic nucleus controls pineal melatonin synthesis and the concentrations of melatonin in the sera of healthy subjects, which reach values of 10⁻¹⁰ to 10⁻⁹ mol/L during the night, with much lower concentrations being present during the day. Many publications have shown that melatonin has an important role in a variety of cardiovascular pathophysiologic processes: the indoleamine has anti-inflammatory, antioxidant, antihypertensive, antithrombotic and antilipaemic properties1 (figure 1).

In their Heart manuscript, McMullan et al2 report their findings when analysing data from the Nurses's Health Study I (NHS I) and NHS II to investigate the independent association of nocturnal melatonin secretion with the incidence of myocardial infarction (MI). In accordance with the authors, the NHS I began in 1976, with 121 701 registered nurses aged 30–55 years returning an initial questionnaire; the NHS II began in 1989 with 116 430 female registered nurses. In the initial and subsequent biennial questionnaires, health status, medications, dietary intake and lifestyle factors, including smoking history, physical activity and sleeping patterns, were ascertained. In addition to completing questionnaires, blood and urine samples were provided by 18 743 women between 1999 and 2000 in NHS I …