The term “circadian rhythm” refers to your body’s internal clock, or the natural way the body governs daily processes— everything from the secretion of enzymes and hormones to when you fall asleep and when you wake up. These rhythms are often set by environmental cues; for example, your body starts to wind down and prepare for sleep after sunset, and it’s prone to wake up at sunrise.

Maintaining an optimal circadian rhythm is hugely important for hard-training guys as any disruption in the body’s natural rhythms can in turn disrupt the production of anabolic hormones like testosterone and insulin, and rob you of the energy you need for intense workouts. So not only is it important to get rest when your body tells you to, researchers at Vanderbilt University showed in a recent study that it’s fairly easy to pack on fat if you ignore the circadian rhythm. Specifically, researchers discovered that insulin sensitivity is in part governed by circadian rhythms and you’ll be less sensitive to insulin as your body enters the inactive/fasting phase near the end of the day, and hence far more likely to store calories as fat.

The researchers said this helps explain increased obesity and diabetes rates amongst people who work the night shift and recommended that people avoid eating altogether between dinner and the following day’s breakfast.


Circadian rhythms aren’t entirely dependent upon the rising and setting of the sun, but your body is naturally inclined to follow that cycle. Recent studies have shown that the body is best primed for exercise in late afternoon— between 4 and 5 pm—when body temperature is naturally at its highest point. The studies showed that both strength and endurance are about 5% higher in late afternoon, and you’re less likely to suffer an injury when training at that time.

While late afternoon might be the optimal training time for for most guys, it doesn’t necessarily mean you should abandon the morning routine if that’s when you typically train. Separate studies have shown that guys who train in the morning are more likely to stay consistent in the long run.