People often ask why training in the evening is such a bad thing. Wait, is it? Yep. It has to do with cortisol production. Training spikes cortisol. So let's look at what cortisol does.
Its main function is putting your body in the best possible state to face danger or stress. It mobilizes stored energy, increases wakefulness, and shuts down the immune system momentarily so you have more energy for the muscles and organs, which are needed to face the stress. It also amps up the brain, mostly by increasing the conversion of noradrenaline to adrenaline.
The ideal cortisol cycle is high in the morning and low in the evening. The cortisol spike in the morning is what makes you wake up (when you wake up on your own). The cortisol spike also increases adrenaline levels, which assist in waking you up too.
Then, as your cortisol decreases in the evening, it puts your autonomous nervous system in parasympathetic mode – also known as rest-and-recover mode. That allows you to fall asleep more easily, recover better, get more time in deep sleep, and have a higher production of growth hormone.
If your cortisol stays elevated in the evening, it'll be much harder to fall asleep and get quality deep sleep. That's why training in the evening isn't the best choice.
Let's say you do train at night regularly and have restless sleep as a result. This may lead to chronically elevated cortisol, which is bad for your gainz, bro. First because cortisol increases protein breakdown. The amount of muscle you build is a function of the difference between protein synthesis (anabolism) and protein breakdown (catabolism). If you break down more it'll be harder to add muscle tissue, especially if you're a natural lifter.
Then there's the impact on myostatin. Myostatin is a myoprotein that plays a role in how much muscle your body will allow you to carry. The more myostatin you have, the less muscle you can build. Well, cortisol can increase myostatin and inhibit muscle growth. It also decreases the rate of muscle glycogen resynthesis – storing glycogen in the muscles after you used it up during training – delaying recovery.
For all of these reasons, the more you can spike cortisol from training in the earlier part of your day, the more you can respect the natural cycle of your body and the better you'll recover.
Enhanced lifters have fewer problems because steroids decrease the action of cortisol significantly. But natural lifters need every advantage they can get.
Now, some people can actually pull evening training off: those who fall asleep easily even if they've had a killer workout two hours prior. Normally these guys have either a high level of GABA or a high level of serotonin, allowing them to shut their CNS down as soon as the workout is over, putting them in parasympathetic mode.
Ingesting your carbs in the evening (post-workout) can also help lower CNS activation and decrease cortisol if you train at night.