HYPOTHESIS: Research confirms that our muscle strength is lowest in the morning and greatest in the late afternoon (5 PM - 7 PM). So if you consistently train in the late afternoon, you should be able to get bigger and stronger than if you consistently train in the morning.
RESEARCH: In a University of Jyväscylä (Finland) study, subjects followed a 20-week strength-training program that worked the whole body every workout, with special emphasis on the leg muscles. The first 10 weeks served as a preparatory phase to acclimate the subjects to weigh training. During this phase, the subjects trained just twice per week in the late afternoon. For the final 10 weeks, the subjects were placed either in a morning or late-afternoon training group. During these final 10 weeks, training frequency was gradually bumped up to three times per week.
FINDINGS: The researchers reported that muscle size of the legs increased similarly (by about 3%) regardless of the time of day that subjects trained. The results were also similar for strength gains, with both groups making equivalent increases in muscle strength.
CONCLUSION: Although some research studies have shown that training in the afternoon can lead to slightly better gains in muscle mass and even enhanced fat loss, this study suggests that the time of day that you work out has little impact on your results.
APPLICATION: The take-home point here is to simply train when it best fits your schedule. That way you are less likely to miss workouts and training consistently is what produces the best results in strength and muscle mass gains.