Sometimes we see the brain and the body as separate entities. They are not.  The human body is an integrative machine that rarely excludes things. Even when you are training alactically, there is an aerobic aspect related to your recovery. We are now beginning to understand the impact of mental tasks on total body fatigue

Science Daily bring us insight into how this beautiful integration happens. The pre frontal cortex is a part of the brain that is activated when mental and physical tasks coexist. Meaning that it is active pretty often. In the sliding scale of cognitive and physical the vast majority of activities will fall somewhere in the middle regardless of what side they favor, there are few outliers.

"study findings show that there were lower blood oxygen levels in the PFC following combined physical and mental fatigue compared to that of just physical fatigue conditions. Through simultaneous examination of the brain and muscle function it is apparent that when participating in highly cognitive tasks, brain resources are divided which may accelerate the development of physical fatigue."

Now this doesn't mean you should be avoiding using your brain because it will hurt your squat. If anything using your brain more often would offer benefit. However it does lead us to understand that there is only so much stress we can cope with. The pre frontal cortex doesn't have an endless supply of energy to function. With its constant usage, highly demanding tasks like a big project at work or a very difficult training session can be equally as taxing. Be conscious of the stressors in your life and logically plan how you deal with them.

I highly encourage you to read the full article.

There are an array of supplements from BCAA's to herbal that can help cope with stress.