http://www.nytimes.com/2009/12/10/health/nutrition/10best.html?_r=1&ref=nutrition

Just about every fitness book has a little blurb in it saying, "When's the best time to train?" which winds up concluding, "Whenever you can get to the gym on a regular basis." They weigh the pros and cons of early-bird and night-owl-dom and conclude that there's no difference--there are advantages and disadvantages to both.

I've always been a morning exercise guy, but that's just me. Some people are useless in the morning, both in the gym and elsewhere...believe me, I've trained some of these people, and I know. 

The New York Times, however, suggests that there actually is a correlation between time of day and neuromuscular efficiency, strength and power, which suggests, counterintuitively, that to perform your best, exercise later in the day is preferable. This has to do with being more warm and springy, having less fluid in your spine, even with greater cardiovascular efficiency: your heart rate can actually go higher in the evening. On first glance, this would seem to suggest less efficiency--ie, that the c.v. system is working harder to produce the same work output--but according to the article, the opposite is true. The system is just more responsive:

...a small group of researchers has studied the question of exercise performance and time of day, even doing studies of heart rates. And not only are performances better in the late afternoon and early evening, but, contrary to what exercise physiologists would predict, heart rates are also higher for the same effort.

One recent study...found that people’s maximum heart rates and sub-maximal heart rates were lower in the morning but that their perception of how hard they were working was the same in the morning as it was later in the day.

[Researchers]... also noted that athletes’ best performances, including world records, were typically set in the late afternoon or early evening.

Greg Atkinson, also at Liverpool John Moores University, said that some researchers, noticing that heart rates during exercise were lower in the morning, reasoned the way I did — that people must be more efficient in the morning. It would mean that exercise was easier in the morning. Of course, it seemed harder to me, but I could have been deluding myself. Not really, Dr. Atkinson said. It actually is harder to exercise in the morning.

"Most components (strength, power, speed) of athletic performance are worst in the early hours of the morning," he wrote in an e-mail message. "Ratings of perceived exertion during exercise have generally been found to be highest in the early morning."

I can't remember a time when I wasn't a morning exerciser by preference. And triathlon, in which I've participated since 2005, is a seriously morning sport: start times can be in the 6 AM hour, god help us. But my max heart rate is also way, way lower than the typical calculations suggest it should be: 140 beats per minute for me is tough, and if I do uphill resisted sprints, or some other similarly max-effort work,160 is about the highest I can get to, even though my max heart rate should be closer to 180. 

I'd like to think that this was an indication of my Lance-Armstrong-esque fitness level, but I think it's just a physiological quirk. My resting heart-rate--a pretty reliable indication of fitness--is just under 60 beats a minute, meaning 'healthy' but not a superman. 

As to the question of the best time to train: sounds like these studies are pointing towards afternoon or evening for best performance, though personally I'm still going to work out in the morning, even if it means I fall a few percentage points shy of my personal best on those days. I like that anyway: it means I can justify that extra 10% I always tag on to my numbers when people ask me my personal best on various lifts.