Records are usually
set in the afternoon
or evening.


By GINA KOLATA

Are some athletes more efficient depending on what time of day it is?

Tara Martin, a triathlete, said she could never get her heart rate up in the morning.

Richard Friedman, a swimmer, said his heart rate was always lower in the morning. His swim team does the same workout in the morning as in the evening, and he swims it just as fast. He had assumed that somehow he was just not putting in the same effort early in the day.

��Still,�� he said, ��I��m pretty energetic all the time.��

I asked Dr. William Haskell, an exercise researcher and emeritus professor of medicine at Stanford University, if I��d stumbled on a known fact about heart rates after researching my own exercise patterns: high heart rate at night, low in the morning for the identical workout. But he was baffled.

Dr. William Roberts, a former president of the American College of Sports Medicine and a family physician at the University of Minnesota, said it was a ��tough question.���� He added, ��I do not have a good physiologic explanation for the phenomenon you are describing.��

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, 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.

Dr. Reilly and his colleague Jim Waterhouse, in a review published this year, 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 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.��

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.��

��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.

In fact, Dr. Smolensky added, people at risk for a heart attack should plan their workouts for late afternoon or early evening.

��My personal approach is to train when your biological efficiency is greatest, which means late afternoon or early evening for most people,�� he said. ��Others say if you train when your biological efficiency is least you will get a harder workout.��






A workout in the morning raises the heart rate less than an equivalent one later in the day. / PHOTOGRAPHS BY FILIP KWIATKOWSKI FOR THE NEW YORK TIMES