It’s almost a willing choice on the part of your body. In a million little ways your body can choose to operate in “two” different modes:
(“two” actually means a multidimensional spectrum, a huge vector of floating point values)
Your body’s got High-Alert mode, which involves being ready to fight, ready to hide and strategize, but which causes more breakdown of tissues and organizational systems …
Or it can operate in Low-Alert mode which is less ready to fight, less safe in a dangerous environment, but also causes less degradation of the systems.
Ship at full battle ready — every engine turned on and hot, every sailor at his station, versus a ship at normal daily duty: some systems turned off and being repaired, some sailors snoozing in their bunks, eating.
High Alert mode is stress, basically. Or technically it’s the response to more stressors. You go into High Alert mode and you drain resources faster.