Interesting. I cycle. This year I put in over 2000 miles. I've learned the hard way that if I push too hard for too long, I'm not going to get a good night's sleep. I've not read about a link between overreaching and increased cortisol levels. Can you provide this author's name and perhaps suggest some further reading that describes this process?
Sure. I'd recommend checking out the Primal Fitness Guide at Mark's Daily Apple. It's a free download if you register for the site. The author, Mark Sisson, is a former Olympic marathon runner and triathlon junkie who has shifted his approach to exercise in part because of cortisol-related stuff.
http://www.marksdailyapple.com/
You could also go to iTunes and sign up for the Paleo Solutions podcast by Robb Wolf. If you start from the beginning with the early podcasts, you'll hear Robb talk at length about his efforts to get his top-performing CrossFit and other athletes to rest more and dial it down a notch or two. Once he gets their cortisol issues under control, he says, they tend to feel and perform better.
Robb talks a lot about the importance of sleep as well. I would not recommend the book
Light's Out by T.S. Wiley, although Robb Wolf sometimes recommends it. To me, this book is poorly written. However, it is full of NIH and other research about sleep, cortisol, insulin-resistance and the current epidemic of obesity, diabetes, CVD, etc. Lastly, on the diet issue, run, don't walk, to read Gary Taubes's tour de force,
Good Calories, Bad Calories. (Sorry if I'm trying too hard to "lay my trip" on folks!)