About 325,000 people in the United States suffer sudden cardiac death each year.
Heart attacks are more commonly seen during early morning and to a lesser extent in the evening hours; though can occur at anytime of a day.
The body clock, circadian rhythm might have a role to play in it  The first molecular evidence linking circadian rhythms has been suggested in a recent study. Read more; http://clinispot.blogspot.in/2012/02/circadian-rhythm-and-heart-attack.html