Some extracts from this article by Mark Sisson..
Performance
If you're interested in performance, some evidence seems to point toward the afternoon and early evening as the best time to exercise. In one recent study, ten and eleven year old boys performed better in the afternoon than in the morning. Grip strength improved 5.9%, squat jump performance increased 3.5%, the “5 jump†test improved 5%, and performance increased 5.5% (peak) and 6% (mean) during the Wingate test (which measures peak anaerobic output) from morning to afternoon. They also tested the boys in mid afternoon (2:00 PM) and early evening (6:00 PM), but found no difference in performance. This study found a similar result in morning and afternoon PE students using the Wingate test, but the authors suggest that longer warmups during morning sessions could mitigate the performance deficit. So, morning workouts require longer warmups? I can buy that.
Sprinting? Try afternoons for maximal power, but don't think you'll have any extra stamina. This study found that maximal power was highest during the first three sprints (of ten total) in the afternoon when compared to morning sprints. Fatiguability was no greater, though.
Another cycling study found that afternoon power output was greater than morning power output in trained cyclists. Fatigue was not affected by time of day, however.
Another study compared the knee extensor strength (via the mighty half squat!) of two groups of trainees. The first group had spent 10 weeks training in the morning, while the second group had spent 10 weeks training in the evening. Both groups improved over baseline, but the evening trainees enjoyed greater strength gains.
I wouldn't assume that all exercise is better in the afternoon, however. The authors of this study suggest that submaximal exercise - like jogging, walking, hiking, or anything that relies on stamina and little else, really - is not negatively impacted by diurnal variation, which makes sense in light of the previous studies showing that while maximal power decreases in the morning, stamina does not.
Takeaway: Raw power and performance in weight lifting, cycling, and sprinting is highest in the afternoon and evening. Stamina does not increase or decrease, however, and the changes in performance may be mitigated by more extensive warmups. If you're a high level athlete or really interested in how you perform, afternoons and evenings are best. If you're just trying to get and/or stay fit, strong, and healthy, morning workouts are just fine.
Hormonal Effects
How does timing change how a workout affects your hormonal profile?
One study found that lifting heavy things in the evening, as opposed to the morning, resulted in a more anabolic (strength and size-building) testosterone/cortisol ratio. Baseline cortisol values were lower in the evening (which is normal) and higher in the morning (also normal; cortisol is necessary for morning wakefulness), which probably explains why evening training worked better. If you're starting with already elevated cortisol, some intense training is only going to spike it even more. If you're starting with moderate cortisol, intense training won't have as negative an effect and your testosterone levels will likely be sufficient to counteract the rising cortisol.
Not all research is so clear-cut, though. In a study on muscular hypertrophy (growth), both morning and afternoon weight-training worked equally well at growing muscles and increasing maximal strength. There was a slight, but statistically insignificant advantage to training at night.
Takeaway: Don't hop directly under the bar in your pajamas with bleary eyes. Take the time to wake up and relax before working out. That might mean pushing your morning fitness class to an afternoon session, or at least a late morning one. We know that cortisol is normally elevated in the mornings, and exercise increases cortisol, so be aware of how the two interact.
Performance
If you're interested in performance, some evidence seems to point toward the afternoon and early evening as the best time to exercise. In one recent study, ten and eleven year old boys performed better in the afternoon than in the morning. Grip strength improved 5.9%, squat jump performance increased 3.5%, the “5 jump†test improved 5%, and performance increased 5.5% (peak) and 6% (mean) during the Wingate test (which measures peak anaerobic output) from morning to afternoon. They also tested the boys in mid afternoon (2:00 PM) and early evening (6:00 PM), but found no difference in performance. This study found a similar result in morning and afternoon PE students using the Wingate test, but the authors suggest that longer warmups during morning sessions could mitigate the performance deficit. So, morning workouts require longer warmups? I can buy that.
Sprinting? Try afternoons for maximal power, but don't think you'll have any extra stamina. This study found that maximal power was highest during the first three sprints (of ten total) in the afternoon when compared to morning sprints. Fatiguability was no greater, though.
Another cycling study found that afternoon power output was greater than morning power output in trained cyclists. Fatigue was not affected by time of day, however.
Another study compared the knee extensor strength (via the mighty half squat!) of two groups of trainees. The first group had spent 10 weeks training in the morning, while the second group had spent 10 weeks training in the evening. Both groups improved over baseline, but the evening trainees enjoyed greater strength gains.
I wouldn't assume that all exercise is better in the afternoon, however. The authors of this study suggest that submaximal exercise - like jogging, walking, hiking, or anything that relies on stamina and little else, really - is not negatively impacted by diurnal variation, which makes sense in light of the previous studies showing that while maximal power decreases in the morning, stamina does not.
Takeaway: Raw power and performance in weight lifting, cycling, and sprinting is highest in the afternoon and evening. Stamina does not increase or decrease, however, and the changes in performance may be mitigated by more extensive warmups. If you're a high level athlete or really interested in how you perform, afternoons and evenings are best. If you're just trying to get and/or stay fit, strong, and healthy, morning workouts are just fine.
Hormonal Effects
How does timing change how a workout affects your hormonal profile?
One study found that lifting heavy things in the evening, as opposed to the morning, resulted in a more anabolic (strength and size-building) testosterone/cortisol ratio. Baseline cortisol values were lower in the evening (which is normal) and higher in the morning (also normal; cortisol is necessary for morning wakefulness), which probably explains why evening training worked better. If you're starting with already elevated cortisol, some intense training is only going to spike it even more. If you're starting with moderate cortisol, intense training won't have as negative an effect and your testosterone levels will likely be sufficient to counteract the rising cortisol.
Not all research is so clear-cut, though. In a study on muscular hypertrophy (growth), both morning and afternoon weight-training worked equally well at growing muscles and increasing maximal strength. There was a slight, but statistically insignificant advantage to training at night.
Takeaway: Don't hop directly under the bar in your pajamas with bleary eyes. Take the time to wake up and relax before working out. That might mean pushing your morning fitness class to an afternoon session, or at least a late morning one. We know that cortisol is normally elevated in the mornings, and exercise increases cortisol, so be aware of how the two interact.
It is hard to start a workout but once you get into it you feel keep going!
