What are the physiology and psychology of the phenomenon that people call being "burnt out"? is this a medical illness or disease or is sociocultural?

Mixed bag: I think it is usually intended to be more socio-cultural, but we are complex organisms that are interacting with a complex environment. It's like a vicious cycle and our biochemistry is altered by the stress of being "burnt out". So, it leads to medical disease which increases the stress too. Not a pretty picture. Mindfulness meditation helps some people stay "cool".

Burnout: "burnout" is a state of mental, physical, and emotional exhaustion brought on by excessive and prolonged stress. You lose the interest & motivation that led you to take on the role. Emotions become numb, & you can become cynical & resentful -- feeling you have no more to give. It's not a medical illness, but can certainly lead to that. http://www.helpguide.org/mental/burnout_signs_symptoms.htm.

You are asking a: question that takes much more than 400 characters to answer. There are whole books written on both the physical and psychological components of 'burn out,' which is a lay term not a medical term.

Sameness: Burnout can be thought of as doing exactly the same thing, the same way, every day, day in and day out without variation. Also burnout can be thought of doing too much of the same thing too fast in too short a period of time, which leads to sensory overload, which can be a major aggravating factor in burnout.

More sociocultural: Just a common term usually used to describe an experience when one feels overburdened, fatigued, overly stressed.