A new study performed by researchers of Indian-origin is all set to answer your questions. You might have experienced this often. When we do some mental activity such as studying or working on a report that needs a lot of concentration, we tend to feel extremely tired physically. And, this makes us wonder why do I feel tired when I was only stressed mentally?
Ranjana Mehta, assistant professor at the Texas A&M Health Science Centre School of Public Health conducted a small study to analyse the co-relation between mental stress and physical fatigue. The study found that when we get involved in mental and physical tasks at the same time, some specific areas, called prefrontal cortex (PFC) are activated in our brain. This can causes our body to get tired or fatigued much sooner than if we had done only a physical work.
During this study the researchers simultaneously monitored the brain and muscle function. The findings suggested that when we do mental and physical work at the same time the oxygen supply to the brain gets divided, which may accelerate the development of physical fatigue.
If you notice, most of the time we do physical and mental work together using both the body as well as the brain. For e.g., travelling to attend an important meeting. Considering this fact it is very likely for a working professional who faces physical as well mental stress each day to develop physical fatigue.