MY friend Jen Davis and I often run together in the morning because it can be easier to fit in a run before work than after. But we always thought we ran better in the evening.

Then I accidentally discovered something weird. I took a spinning class one Thursday night, and my heart rate, measured by a monitor strapped around my chest, soared. I don’t usually use a heart-rate monitor, but with stationary bikes, heart rate is pretty much the only way to know how hard you are working. And that night, my high heart rate told me it really was a tough workout.

The next morning I did a workout in my garage on a trainer — a device that holds a road bike, turning it into a stationary bike and yet allowing you to use its gears. My heart rate was about 15 beats a minute lower than it had been the night before. It seemed like a pitiful workout.

So the next night I got on the trainer again. I had the same playlist (I use music to set my cadence). I used the same gears for each song. And during the hourlong workout, my average heart rate and my maximum heart rate were about 15 beats a minute higher than they’d been the morning before.

I tried again the next morning. My heart rate was low. Intrigued, I tried my experiment for a week, alternating between early morning and early evening workouts. I got really sick of that playlist, but I wanted to control every variable.

And the pattern persisted: high heart rate at night, low in the morning for the identical workout. Once I even tried the workout in midday — that time, my heart rate was in between.

Could it be that I actually was a more efficient athlete in the morning, doing the same work but with less effort, as measured by a lower heart rate?

Jen reminded me that we’d seen the heart-rate effect last year but had not appreciated it. I had a stress fracture and was confined to pool running, which involves sprinting in the deep end of a pool. Your feet never touch the bottom. It was hard to gauge how hard we were working, so Jen and I wore heart rate monitors, just as we do in spinning classes.

We did the pool workouts together, and neither of us got our heart rates as high as we wanted in the morning. Evenings were fine, though. We thought we were just sluggish in the morning.

I also asked some friends who use heart rate monitors if they’d noticed anything like what I’d experienced.

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, if I’d stumbled on a known fact about heart rates. But he was baffled. Maybe I didn’t have caffeine in the morning? So I tried taking NoDoz before the next morning workout. It made no difference.

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.”

But, it turns out, 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 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.”