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.