Everyone marches to a different rhythm, which is why your friend can pop out of bed at the crack of dawn for a fifty-mile ride and you can't bear the thought of lacing up your running shoes until at least three o'clock in the afternoon.
The reason for this, at least in part, is a complex network of body clocks called circadian rhythms. These timekeepers control everything from hormone fluctuations to sleeping patterns. "The main body clock is located in the hypothalamus region of the brain," says Alex Zambon, Ph.D. a post-doctoral fellow with the Gladstone Institute, a medical research center at the University of California, San Francisco. "It communicates with a series of smaller, peripheral clocks that keep time in other places like your liver, heart, and skeletal muscles."
Experts have long thought circadian rhythms can have a powerful influence on your workout. For instance, your body temperature is at its lowest right before you wake up in the morning, making a.m. the optimal time of day for moderately paced endurance activities like jogging and low-impact aerobics. Cooler body temperatures tend to subdue your body's physiologic responses. For example, your heart rate won't spike as high during a morning workout as it will during a similar afternoon workout so you can give more of an effort with less stress on the body. Studies also show that many exercisers naturally choose a faster pace in the morning as compared to later in the day, perhaps because they're less likely to overheat.
On the other hand, the ideal time to hit the weight room or tackle a high intensity spin class seems to be in the late afternoon, when your body temperature and muscle strength are at their zenith. A higher body temperature seems a key to making your muscles work more efficiently because they are more pliable and quicker to respond. And many people feel better when they do workouts that involve a lot of stretching (like yoga and Pilates) later in the day. This is probably because their joints are looser, although it's not known if stiffness is greater in the morning because people have been in bed for eight hours or because there is a circadian rhythm for stiffness that peaks in the morning.
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