I was really surprised by this article in the NYTimes.com because it is saying that even with the same workout done the same way, your heartrate could be higher in the evening than the morning. The article goes on to discuss why this could be. It’s fascinating in itself, but as someone who trains mostly later in the day but runs early morning marathons and other long distance races, it’s also concerning. I train with a heart rate monitor, and my tempo runs are based on my heart rate later in the day. When I run the race, I am assuming my heart rate is going to be the same for the same effort. But in fact, to get the same heart rate in a morning race, I might have to be running alot faster. This sounds like it could be a good thing, but you could also end up over-exerting yourself and not having enough strength in your legs to get to the finish line.
The net of it is, if you are going to run a morning marathon, do some long distance and tempo runs in the morning as well to have a better understanding of what pace you should be running at come race day.