Circadian variation in vascular tone

To the Editor: The possibility of a circadian variation in the incidence of stroke, myocardial infarction, and sudden death has intrigued investigators in recent years.^{1 2 3} Although some workers have implicated the sympathetic nervous system in this purported temporal pattern of heart disease, others have dismissed such an association. The reasons include the findings that catecholamine secretion does not increase during spontaneous episodes of ST-segment elevation⁴ and that ischemic changes in ST waves during early-morning sleep are not preceded by increases in heart rate and do not appear otherwise to involve increased adrenergic activity.⁵ The report by Panza et al. (Oct.