"My experience with training athletes, as well as with my own training, has been that people naturally train better when their cortisol levels are high. Since cortisol levels rise with the sun, reaching peak blood levels around 9-11 a.m. and then progressively set with the sun, most of you will find that you get your best performances in this timeframe.
If your schedule doesn't permit you to train at this time, at least you can set your schedule so that your hardest workouts are on weekends or your days off from work, allowing you to train with your natural cortisol tides.
For those of you who currently wake up in the morning feeling tired - even after sleeping eight hours - training in the evening after work may well be disrupting your sleep and recovery cycles.
This is because performing any exercise that is more intense than you could perform on a full stomach triggers the release of cortisol, telling your body that it is some time between sunrise and about noon.
There's a good reason why we're built this way. For thousands of years, if not millions, we did our hunting and gathering from sunrise until just before noon. When you elevate your cortisol levels at night by hitting the gym after work, you literally wind yourself up! Since cortisol lasts for hours in the body before it is used up or neutralized by the liver, it will stop you from getting a deep, restorative sleep." -Paul Chek
Morning or Night Person