Better to exercise in the morning, or later in the day?

Personal Best – To Exercise Early or Late, Now There’s a Question – NYTimes.com

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 certainly  find this to be true.

“Is a heart rate of 140 in the morning indicative of the same level of workout cost as in the afternoon?” asked Dr. Smolensky, a visiting professor at the University of Texas Health Sciences Center in Houston.

“I would say no,” he added. “Exercise physiologists say you should be able to perform at the same level with a heart rate of 140 in the morning as in the afternoon or early evening. But chronobiologists say your capacity to generate and tolerate a higher heart rate is better later in the day.”

“In the afternoon and evening,” Dr. Smolensky said, “you are in a different biological state.”

But, he added, all this applies to people who are regular exercisers, who work out vigorously three or more times a week. People who are not regular exercisers, Dr. Smolensky said, put much more strain on their hearts in the morning, making their heart rates higher then.

The article goes on to say that if you are at risk of a heart attack, it's better to work out in the afternoon or early evening.