Another reason to exercise in the morning: You work harder.
[N]ot 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, by the late Thomas Reilly and his colleagues at the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University in England, 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.According to studies cited in this New York Times article, "[i]t 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.”
If you exercise later in the day, your muscles are more flexible and stronger and your heart and lungs are more efficient, said Michael H. Smolensky, an expert in chronobiology, the study of the body clock.
“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.”So set your alarm clocks, people.