From Charles Poliquin
You know instinctively that the best time to train is whenever you feel most energetic and can fit it into your schedule. Individual differences regarding the best time of day to train come from the fact that each person has slightly different chronobiology, or circadian rhythms. Research into the area reveals some noteworthy points:
• In performance that relies on the anaerobic system, such as strength training, studies suggest strength performance is significantly greater in the mid-afternoon and early evening. This effect is most pronounced in regards to power production—when needing to produce maximal force at a fast speed.
• For sprint training, competing or working out in the mid-afternoon and early evening tend to produce faster times. Greater strength and speed later in the day are likely due to the fact that your body temperature increases as the day progresses, leading to enhanced muscle function and flexibility.
• For fat loss, there’s evidence that trainees experience a greater afterburn (EPOC) if they work out in the evening around 6 pm. Higher EPOC means the body burns more calories during the recovery period. Meanwhile, for endurance training, studies show time of day doesn’t matter much for performance, but of course individuals will have distinct experiences.
• Of interest, it’s well known that for men, testosterone is highest in the morning and gradually declines over the course of the day, indicating that for building muscle, morning may be the best time to lift. In addition, the male testosterone response to training appears to be greater in the morning than from training later in the day.
• If you’re interested in using caffeine to offset the poor performance decrement of morning training, check out the tip, Use Caffeine to Perform Better in the Morning.
Deschenes, Michael. Chronobiological Effects on Exercise. ACSM Current Comment. Retrieved 25 April 2013. http://www.acsm.org/docs/current-com...onexercise.pdf