My wife's first words to me this morning were "You shouldn't run in the morning, it increases your risk of stroke" - in the background a man in a white lab-coat gesticulated on the TV, but I didn't stop to listen*.
The reason for this permutation of the words "good morning, treasured soul-mate" is my recent foray into exercise, particularly the morning run which I've incorporated into my week in an attempt to make my body more resistant to the risk factors incurred by my inability to peel myself away from a computer screen.
In particular, I've gone through the podcast-based "C25K" training program, with some minor alterations**. My main aim here was to bring my resting heart rate to a more reasonable level (I measured 90 or so, which is right on the upper limit of normal), so I recorded it the whole time I was doing the program.
For the record, heart rate is a pretty strong indicator of cardiac risk, and being at the high end is probably not what you want, especially when you factor in the proportion of deaths due to cardiovascular problems in developed countries.
So what did I find? The C25K program was effective at reducing my heart rate. I'm not sure by exactly how much, however, due to my somewhat inconsistent method for taking my pulse (I became more fussy about when I took my pulse in the latter half of the program, which is probably a big source of bias considering how much my pulse changes during the day). Nonetheless, maybe I reduced my pulse by 10 beats per minute or so, 20 beats per minute at the most.
Using the diagrams on the previous link ("High heart rate: a cardiovascular risk factor?" - Cook et al. 2006), that corresponds to a reduction in risk of heart failure by anything up to 50%. Given the reduced cardiovascular risk, I'm fairly sure I'm better off running, and risking a morning stroke, than not running at all. I cant run in the day because it is simply too hot***, and I don't fancy running in the evening on a belly full of dinner, or in the dark*4*.
While I had a hard time finding any evidence to the contrary*5*, I just managed to dig up this paper, which states that "the protective effects of exercise were more significant in the
afternoon and evening group than in the morning and forenoon group". This sounds fair enough to me: its not a bad idea to run in the morning, but its likely a better idea to run in the afternoon.
*TV/radio knowledge is somewhere behind oral tradition in my mental filing cabinet. In my mind, anything heard on a program not even ostensibly about science means nothing more than something to perhaps look up on the internet later. Of course, the internet can be far, far worse, but at least you can find peer-reviewed science on it.
**specifically, I juggled around the order of the runs slightly so that the length of the runs increased more linearly. Those familiar with C25K will remember looking at week 5 and thinking "WTF!?". I recommend juggling around the days as you see fit: the person who made C25K doesn't appear to have had any formal health/exercise qualifications at the time he designed the program, so I can't imagine you'll do yourself any additional harm by applying logic, especially since it will make the progression less intense.
*** To me, this sounds like a damn good way of getting heat stroke.
*4* And this, a pretty good way of distributing my brain on the bottom of an irrigation ditch. Morbid? Me? Noo...
*5* Maybe more due to lack of familiarity with the subject than an actual lack of evidence