Heart Attacks and Strokes are caused by blod clots.  They happen first thing in the morning because the increased heart rate frees the clot to travel and get stuck.

I had my stroke at 1am, early in the morning, but at the end of a long day, just after I had pulled out of my driveway to pick up a snack. 

It's less important when you drink water as how much you drink per day.