Thanks for dragging up that study, Slackers. I'm somewhat limited in ability to read SCIENCE, but I'm taking away this:
"In conclusion, when the confounding factors of meals, prior exercise, and sleep are controlled, time of day has no effect on the magnitude of the Growth Hormone response to exercise. In contrast, the cortisol response to exercise is modulated by time of day. Peak cortisol concentrations in response to exercise were highest at 0700 h, followed by 2400 and 1900 h in that order. In contrast, maximal increases in cortisol concentrations over time in comparison to control day conditions occurred at 2400 h, followed by 0700 and 1900 h, in that order. These data suggest that the circadian rhythm of cortisol secretion influences the cortisol response to exercise."
James, I too always feel dead weak if I try to train strength in the morning, but cardio is fine. I can do volume/low-intensity bodyweight stuff like press-ups or pull-ups in the morning but am definitely slightly weaker than at other times of day. I don't like early starts on climbing trips either...
Regarding the effects of cortisol I found this:
"Small increases of cortisol have some positive effects:
A quick burst of energy for survival reasons
Heightened memory functions
A burst of increased immunity
Lower sensitivity to pain
Helps maintain homeostasis in the body
Higher and more prolonged levels of cortisol in the bloodstream (like those associated with chronic stress) have been shown to have negative effects, such as:
Impaired cognitive performance
Suppressed thyroid function
Blood sugar imbalances such as hyperglycemia
Decreased bone density
Decrease in muscle tissue
Higher blood pressure
Lowered immunity and inflammatory responses in the body, slowed wound healing, and other health consequences
Increased abdominal fat, which is associated with a greater amount of health problems than fat deposited in other areas of the body. Some of the health problems associated with increased stomach fat are heart attacks, strokes, the development of metabolic syndrome, higher levels of “bad” cholesterol (LDL) and lower levels of “good” cholesterol (HDL), which can lead to other health problems".
So it sounds like exercising late can mean that you have more cortisol in your body over a longer period of time, which could contribute to the negative effects described above. I guess as well as getting a shorter night's sleep, which also 'ain't great for recovery.
Hmmmm...
My current evening session would involve getting to the wall for 6.30ish, warming up/doing easier problems for an hour, doing two hours of bouldering, then half an hour or so of endurance circuits. I leave when it closes at 10 (try to eat a post-workout protein bar), and get home around 11 (yeah I live in London...) Make a snack (egg on beans on toast usually), don't get to bed til 12.30, get up at 7.
Maybe I'm letting myself 'av it and need to do a shorter session so I get to bed earlier. Maybe that will mean less injuries...