J. A Panza, S. E. Epstein, and A. A. Quyyumi, “Circadian variation in vascular tone and its relation to alpha-sympaiheric vasoconstrictor activity,” New England Journal of Medicine 325 (1991), pp. 986–990; P. M. Ridket et al., “Circadian variation of acute myocardiat-infarction and the effect of low-dose aspirin in a randomized trial of physicians,” Circulation 82 (1990), pp. 897–902.