The human body has built-in clocks. Specific body processes occur each day at about the same time. The science that studies these natural body rhythms is called chronobiology, and scientists in that field have discovered some fascinating things about the body’s rhythms that directly affect exercise, diet and health.
The master clock is located in the brain and is thought to be affected by light and darkness. Certain events occur during the day, others at night. An example is hormone secretion. Growth hormone release peaks during the initial 2 1/2 hours of sleep, a stage of sleep known as stage 4 that occurs before the onset of dream sleep, known as REM, or rapid-eye-movement, sleep.
Both testosterone and cortisol peak in the early morning, gradually declining as the day progresses. Studies show that drugs may provide greater benefits, with less chance of side effects, if taken according to a specific circadian, or 24-hour, cycle. Aspirin, which is most often taken to treat common pains, such as headaches, also helps protect the cardiovascular system. Specifically, it may help prevent the formation of the arterial blood clots that precipitate heart attacks. The body is most susceptible to internal clotting in the early morning, which is also when aspirin is most easily absorbed. Thus, the best time to take aspirin for cardiovascular protection is early in the morning.
The circadian rhythms of the body also affect exercise. A recent study compared the effects of training in the morning to those of training in the evening.1 Thirteen men, average age 21, who’d trained for at least one year using typical weight-training workouts designed for bodybuilding, trained on an eight-station machine at 6 a.m. and 6 p.m. They did all exercises for three sets of eight to 10 reps using 75 percent of one-rep-maximum weight.
The study found that training in the early morning led to a significantly higher testosterone level during the workout, which makes sense since testosterone peaks at that time. Training in the evening led to a lower cortisol response and a higher ratio of testosterone to cortisol at the end of the workout. That finding led the authors to suggest that the best time to train for maximum anabolic effects is in the evening.
Some of the study subjects, who were used to training at night, experienced increased stress symptoms, characterized by nausea and vomiting, throughout the morning workout. Their bodies had become accustomed to later workouts, and the sudden change led to body-chemistry stress. The researchers didn’t include afternoon workouts; I’ve always had my best workouts between 1 and 3 p.m. My energy seems to peak at that time, as does my mental focus. Training in the afternoon offers some of the same benefits as morning and evening training. I’ve seen other studies showing that muscular strength in most people peaks at about 4 p.m., which is consistent with my experience and that of many others. IM
1 Bird, S.P., et al. (2004). Influence of circadian time structure on acute hormonal responses to a single bout of heavy resistance exercise in weight-trained men. Chronobiol Int. 21:131-146.