Sleep deprivation also led to an increase in the activity of the sympathetic nervous system, which controls many of the body's "involuntary" functions.
People under harmful stress lose the ability to re-engage the parasympathetic nervous system, which drives the body's day-to-day natural functions, including digestion and sleep.
Its functions are vital to the regulation of the body's "circadian rhythms", whose powerful clock-like mechanisms synchronise the body with the 24 -hour period that is day and night.
Obviously, there is more to it, because no one chemical in the bodyfunctions in isolation, and other factors from a person's life experience play a role as well.